Listen to Fromundaman. N-air out of shield is a good option if:
1) The enemy is very near to you (such as after they Dash Attack your shield)
2) You cannot B-air out of shield because of your Facing.
3) You want to deal more damage because B-air/F-air is stale.
4) You don't want to stale out your B-air (e.g. your N-air will knock the opponent off-stage and you can edge guard with a fresh B-air)
5) finally, a grab would be imprudent (due to punishment, unable due to Facing, etc).
Once you work N-air into your game you can use it as a tool to refresh other attacks with an Aerial that can deal 12%, has long Duration (in cases of Air Dodges or Side Steps), and short landing-lag.
Note, it has a terrible Auto-Cancel window due to it's prolonged Duration. However, it can be Landing-Canceled (touch the ground ASAP), of which it's landing lag is better than F-air, and D-air (but not U-air and B-air).
Friendly Reminder: The N-air is usually a poor option to use off-stage. Lol!