I think dair's MOST important use is the Dair-Nair edgegaurd against characters who have trouble sweet spotting (Falcon, Marth, Ganon, sometimes sheik). If you don't know what I am talking watch Armada play against a falcon or something.
Other minute situational uses of dair (that are rather useless against good people) that I have found:
1. Peach Dittos - Dair is actually really good move against Peach lmao. It can catch another peach in her float and combos into many other moves.
2. Recovering - If you are really high in the air and recovering. Dair is a really good move to catch opponents who like to over extend to hit you. You can really turn a situation around with Dair->Bair when recovering. DONT spam it though, its really easy to avoid and abuse.
3. Full hop dair -> float -> nair: If you just wanna troll. Have at it. If you get this, I guarantee your opponent will rage quit.
4. Dair for shield poke -> if you are floating above them and they shield but their head is sticking out, you can use it here. This is a rather old school peach technique.
5. (While on platform and opponent underneath) Low float -> Dair: I mean it works sometimes, but why use this when you can just Dsmash lmao.
6. FC Dair -> Dsmash: I'm not convinced this a legit combo lol.
7. Single hit dair vs fox/falco horizontal upB or side B. Since they are fast fallers, it can act as a psuedo spike if your opponent isn't paying attention.
8. offstage dair -> nair against fox/falco in upB (AKA the Peach shine spike (AKA the I just made up that name right now)): You don't do this unless your tag is Bladewise.
Ask and thy shall receive.