Aerie Sentinel has a more niche role in the deck, which is a weird thing to say for the classic 2 cost, 3 / 4 statline. It is largely because Aerie Steward steals the show as the core unit in the faction. However, Aerie Sentinels are still key to help with two Skyspear Avians needs:
The first is having a solid combat unit that doesn’t rely on extremely strict positioning. As with all 2 cost, 3 / 4 statline units, you can freely summon them and send them around since it’s both decent damage and life for the cost.
The second role is to protect Ava. Ava's major fear is all the Summoner Wars tricks to remove summoner protection which then gives access to the opponent, in this case her own fragile and very valuable life. Aerie Sentinel prevents all that trickery, giving Ava a very critical, solid defender.
Despite this, don't be afraid to use Aerie Sentinels to be part of the flock fighting in melee, especially with the help of Aerie Sages. Aerie Sentinels are always decent summons at least, thanks to that dependable statline, and with Aerie Sages they can become overpowered.