Yeah, besides for edgeguarding, it can be useful if someone is shield pressuring from behind you, cause your options are pretty much limited to roll/spot dodge, WDOoS, or doing an aerial, and dair is the best aerial for this situation in my opinion. It can also be good for combos in some situations. Just be sure you got L canceling down if you're gonna use it on the stage, cause the dair lag time is horrible if you don't L cancel it. As far as edgeguarding with it, I think it works by far the best when done with ken combo. If you knock them off the stage and just wait for them to recover close to the stage, then try to jump off and spike them, you're probably going to suicide a lot even if you pop up right before you dair them (im not sure what the term for that is, where you jump immediately before using an arial when you're off stage), cause the lag time for his dair is so long. So i feel like comboing with a dair spike (like the ken combo) is the best way to off stage dair without risking suicide.