Strong hit combos into Bouncing Fish, Needles, and it can reset neutral. And Sheik has the second best neutral in the game, after Diddy.
Edit: It is the fastest aerial OoS, aside from Sheik's Fair, but ever since the nerf, Nair has been better. It also works as a combo breaker, as it has a much larger hitbox than Luigi's Nair, and Weak Nair is the best weak Nair in the game. Staling the move is actually beneficial, unlike other moves, as the less damage and knockback can extend Sheik's combos. Also, a retreating Nair OoS is safe on shield (Depends on the character, but it works on all but the very fastest). Percentages are very tight, but the combos it leads into can deal 20%+ damage, and it's the best "get off me" move in the game. As I said before, Sheik has good aerials, but her Nair is the best. ESAM and Zero both consider it to be one of the top 10 moves in the game, and they are famous for not agreeing on everything. Although using that particular fact is a weak argument, it does show that top players view Nair favorably. Also this:
https://imgur.com/P3z0EL6