I like to use Nair when falling down against foes since we'll trade and they get hit by the late powerful hitbox. It's also good for getting out of combos and weird approaches your opponent wouldn't aspect, but it's a hard move to optimize.
I kinda want to talk about Dsmash from the previous topic because I think this move has more utility than we think. Dsmash is frame 5. That's ridiculously fast. Like, Think of this; you Baired something's shield and you're baiting an attack. You can walk towards them and Dtilt --> Dsmash frame trap if anyone takes the bait, or with its range and speed, you can surprise someone with this thing offstage and semi-spike-'em to their deaths. There's a lot of utility for this move we're neglecting or not recognizing.
And on future topic, dash attack: dash attack is a option I use as my last resort to catch landings and to frame trap shields with the last attack frames of the move. Besides that, I use it sometimes In Neutral as a random scary move that puts you somewhat above me for Usmash use.