In neutral, dsmash spaced vs say, Fox's aerials leads to a favorable trade, a strong win, or if Fox wins, the extension of your hurtbox through your leg prevents Fox from following up. If he does drill for instance, one ASDI input away should be enough to prevent him from landing any more, and then you can grab his landing. He's also fairly likely to miss his L-cancel if he only lands one hit of drill and it's early.
You can also use it vs Sheiks spacing fairs and a lot of characters spacing aerials. It might lose, but they won't get a followup if they only hit because you stuck out your leg, and a lot of moves are spaced (see: zoning) to prevent you from coming in to begin with. They wouldn't hit you if you stayed stationary or moved back.
Aside from tech-chasing on platforms or at the ledge, dsmash can have enough pushback to knock people onto the ledge, which can be pretty safe. One hit can cover a roll in while the other covers the opponent staying there. Turn around dsmash to change timing can mess with people too, as can charging it. If you've ever played Brawl or Smash 4 Ike, you know running up and charging upsmash is actually a thing. Falcon's dsmash is almost like that. Almost.
Think of Smash attacks in neutral as "high risk, high reward" and work on minimizing risk by spacing them for favorable trades if they aren't to win outright, and implementing ASDI down and other DI as well as what options you'll do next if things don't pan out too peachy. To properly space it vs aerial walls, tilts and other such things, start with a pivot.
