D-air has massively high priority. If you ever get the chance to check out the size of the actual hitboxes in AR, you'll see that out of all of his aerials, it has the largest hitbox. Yes, even larger than b-air/u-air/f-air/sex kick.
The high priority makes the d-air a fairly decent move in all sorts of situations, such as:
1) Fastfalled d-air when in midair can prevent juggling, as it beats out a large number of attacks
2) When shffl'd, it can lead into a reasonable number of different moves to combo: jabs, grabs, u-tilts, shieldgrabbing
3) Shffl'd d-airs at the edge can edgeguard, and especially screw over fastfallers/sweetspotters
4) Fulljumped d-airs/shffl'd d-airs are a reasonable means of approaching the opponent
5) D-air can combo in midair--i.e. u-tilt -> fulljumped d-air -> double-jumped sex kick
One thing to note is that d-airs that land on the ground often have a special hit that causes horizontal knockback; a way to get around this is to combo with d-air at >50%, since at that point, the opponent will be dragged into the air, thus preventing the grounded hit from occurring.
