The comment in Hebrews 2:16 strenghtens the notion of who qualifies as the recipient of Christ's help, and comes on the tail end of the prededing discussion that relates to His incarnation. The structure can be broken down as follows:
1) Jesus was made "lower than the angels" so that He might "by the grace of God [...] taste death for everyone" (2:9-10)
2) Jesus refers to redeemed humanity ("those who are sanctified") as His "brethren" since He (the one "who sanctifies") shares in our (i.e., humanity's + God incarnate's) common origin (2:11-13)
3) Therefore, Jesus "partook" of human flesh & blood so that He might "through death [...] render powerless [...] the devil" and liberate humanity from the fear of death that enslaves us (2:14-15)
This short passage referenced above (Hebrews 2:9-15) can perhaps be viewed as an argument, with the first two points being premises and the third point serving as the conclusion. Taken as such, we can view verse 16 as further expounding upon and clarifying the conclusion of this single "argument." In it, we find a significant element as to not only the purpose of the incarnation, but also its scope (in terms of who Jesus came to "help"). The author makes plain that the work of Christ does not primarily pertain to the angelic hosts, but instead applies "to the descendant of Abraham." This brings us to a question that must be answered properly if we are to understand not only this passage, but, more broadly, God's plan of salvation through Jesus Christ.
Namely, this question is, Who is "the descendant of Abraham"? There is, first, a sense in which this phrase narrowly refers to those who may rightfully claim physical (i.e., biological) lineage, and Scripture abounds with such usage. But this scope is broadened to make available universal application by highlighting Him to whom Paul refers as the "seed" of Abraham. That is, Jesus Christ has enabled descendancy in Himself as being the one in whom the fulness of the promise to Abraham would be realized. In other words in Abraham through Christ "all the nations of the earth will be blessed" (Galatians 3:8-9; Genesis 12:1-3). Speaking to the Galatians regarding the "blessing of Abraham" that "in Christ Jesus" would "come to the Gentiles," he writes:
"Now the promises were spoken to Abraham and to his seed. He does not say, 'And to seeds,' as referring to many, but rather to one, 'And to your seed,' that is, Christ [...] For you are all sons [and daughters] of God through faith in Christ Jesus [...] And if you belong to Christ, then you are Abraham's descendants, heirs according to the promise." (3:16, 26, 29)
Therefore, we find that when taken as a coherent whole, Scripture gives witness to the centrality of Jesus Christ as the key figure through whom the Abrahamic blessing would be bestowed upon all of humanity. Yet, this does not mean that all are automatically "descendants of Abraham" by virtue of being human. Rather, a crucial restriction still holds; Christ fulfills the biological requirement, and we must be in Him in order for the promise to extend to and be realized in us. We are united to "Abraham, the believer" through faith in Christ and, consequently, share in the same inheritance by virtue of Christ and being found in Him (Galatians 3:9).
The extension of this phrase to all of humanity (that is, available to all, but applicable to those who believe) concords with what we have seen so far in this portion of the Epistle to the Hebrews. The whole of humanity has already been in focus prior to this: Christ became "lower than the angels" (i.e., human) so that "He might taste death for everyone" (Hebrews 2:9), "He who sanctifies" shares a common origin with "those who are sanctified" (2:11), and He partakes of "flesh and blood", which is a unifying factor for all of humanity. Thus, it is not surprising that the author in verse 16 makes a distinction between two broad types of entities in the created realm, that is, between the recipeints of Jesus' help and angels (as opposed to, say, making a distinction solely among human persons). The implication must be that the phrase "desendants of Abraham" be taken in broad scope.
St. John Chrsysostom (ad 347-407), Archbishop of Constantinople, expands on this theme in one of the earliest "commentaries" on Hebrews that exists in church history. In his 5th homily on the Epistle, Chrysostom preaches on the nature of Christ's pursuit of humanity by "taking hold" of our flesh and blood, and taking it upon Himself. This pursuit we ought, as believers, to wonder at in amazement if indeed we realize the humility and condescension that characterizes the incarnation of Jesus Christ who suffers everything in order to make us His own. Chrysostom writes that the author (whome he views as Paul):
"[W]ishing to show the great kindness of God towards man, and the Love which He had for the human race, after saying: 'Forasmuch then as the children were partakers of blood and flesh, He also Himself likewise took part of the same' (c. v. 14) — follows up the subject in this passage. For do not regard lightly what is spoken, nor think this merely a slight [matter], His taking on Him our flesh. He granted not this to Angels; 'For verily He taketh not hold of Angels, but of the seed of Abraham.' What is it that he saith? He took not on Him an Angel’s nature, but man’s. But what is 'He taketh hold of'? He did not (he means) grasp that nature, which belongs to Angels, but ours. But why did he not say, 'He took on Him,' but [instead] used this expression, 'He taketh hold of'? It is derived from the figure of persons pursuing those who turn away from them, and doing everything to overtake them as they flee, and to take hold of them as they are bounding away. For when human nature was fleeing from Him, and fleeing far away (for we 'were far off' — Eph. ii. 13), He pursued after and overtook us. He showed that He has done this only out of kindness, and love, and tender care. As then when he saith, 'Are [the angels] not all ministering spirits, sent forth to minister for them who shall be heirs of salvation' (c. i. 14) — he shows His extreme interest in behalf of human nature, and that God makes great account of it, so also in this place he sets it forth much more by a comparison, for he says, 'He taketh not hold of angels.' For in very deed it is a great and a wonderful thing, and full of amazement that our flesh should sit on high, and be adored by Angels and Archangels, by the Cherubim and the Seraphim. For myself having oftentimes thought upon this, I am amazed at it, and imagine to myself great things concerning the human race. For I see that the introductions are great and splendid, and that God has great zeal on behalf of our nature." (Homily V, I Schaff 2004:388)
Since Christ uniquely pursued humanity and "took hold" of our nature, we, in return, inhabit a unique position from which to praise God, offering Him thanksgiving as the recipients of His redemption through the God-man, Jesus Christ. About 1400 years later, John Newton (ad 1725-1807) would write a hymn touching upon this same theme, which still exhorts and encourages us today to uniquely worship the Lamb who was slain for both who He is and what He has done, "for our sake and for our salvation":
Now let us join with hearts and tongues,
And emulate the angels’ songs;
Yea, sinners may address their King
In songs that angels cannot sing.
They praise the Lamb Who once was slain,
But we can add a higher strain;
Not only say, “He suffered thus,”
But that He suffered all for us.
When angels by transgression fell,
Justice consigned them all to hell;
But mercy formed a wondrous plan,
To save and honor fallen man.
Jesus, who passed the angels by,
Assumed our flesh to bleed and die;
And still He makes it His abode,
As man, He fills the throne of God.
Our next of kin, our Brother now,
Is He to Whom the angels bow;
They join with us to praise His Name,
But we the nearest interest claim.
But ah! how faint our praises rise!
Sure, ’tis the wonder of the skies;
That we, who share His richest love,
So cold and unconcerned should prove.
O glorious hour, it comes with speed
When we from sin and darkness freed,
Shall see the God Who died for man,
And praise Him more than angels can
(Now Let Us Join with Hearts and Tongues, from Olney Hymns [written together with William Cowper])
May we join with the saints and angels in that wonderful activity which will forever and willingly occupy our attention and pervade all of our being - praise to the Lord Jesus Christ, to the eternal glory of the Triune God.
Schaff, Philip (Ed.). (2004). Nicene and Post-Nicene Fathers, First Series, Volume 14, Saint Chrysostom: Homilies on the Gospel of Saint John and the Epistle to the Hebrews. Peabody, MA: Hendrickson. (Original work published 1889)