N-Air is the fastest aerial in the game in terms of when it's active and when the animation is over. This makes it solid for disruption and is often super-safe to use as insurance after a ledge jump. If you shorthop immediate N-Air and someone shields it, every character but Mewtwo will miss an immediate shieldgrab, so that could give you time to punish with Up-B, Forward-B, or a landing into something quick.
F-Air sends people forward at a sharp angle; depending on percents and opponents' weight/fall speed, you can get frame advantage even if it doesn't put them into tumble. It can cause people to airdodge, where you can then autocancel into a punish. At high percents it can legitimately be used for edgeguarding.
B-Air is effectively the same move as F-Air but with a hitbox that hits lower (but not significantly so).
U-Air is fast and doesn't send people far, but you have to move backward after landing a hit at low-ish percents or you'll get punished for it. People often get overzealous and it lets you bait swings into your own punish. It also keeps them more-or-less in place for longer, so they panic. It also eats double-jumps from time to time. Not bad if people don't DI away on stray D-Tilts, since you'd rather not stale Up-B before KO percents if you can help it.
D-Air's autocancel window isn't terribly late, so you can use it to protect yourself a bit when falling to the stage. It also lasts for four frames, which isn't bad. It can be used for edgeguards in a pinch against opponents with very high percents who are recovering low, and it's often confusing to an opponent to be hit by shorthop immediate D-Air, so it's not terrible as a risky disruption for a KO Punch or something similar.
Done. There's your metagame.