i like nair for punishing air dodges. if they attempt to air dodge right through you, you will still likely hit.
it's a decent move. it's just that usually a different aerial is better. obviously when the guy is in front of you, fair (or rock smash), bair to punish OoS or cover your backside (and is also your best "approach" aerial), uair to kind of juggle (not really) and punish air dodges. dair is obvious.
nair can edgeguard like fair, but has weak knockback if you don't get the strange sweetspot. nair can punish OoS like bair, but does little knockback and damage, although it has a superior hitbox, so there's a use for it...if you don't want to use one of zard's other **** options (grab, usmash, dtilt, rock smash??, etc). nair can punish air dodges like uair but cannot "juggle", doesn't last as long, and is not as reliable a KO move, but again, nair has a superior hitbox. you can fastfall your uair to kind of achieve the same effect as nair.
so what i'm getting at is if you are in a situation where you have to predict the opponent and want to go for the safe bet, nair is the way to go since it has the best overall hitbox. if a different aerial can work, then you should almost always use that one instead.