this is the short answer.
there are some things in a match that really only find use when you see fit for it. DAir is one of those moves, because it's very bad, but like Bowser Bomb, Firebreath and other moves, it certainly wouldn't hurt you to throw it out sometimes. This is a game where you can afford to do weird ****, and it won't necessarily cause you a game if you're good about it. I'm not saying to go balls out and become a huge nut, but spicing things up sometimes isn't so bad.
here's the longer answer:
it's really bad so if you're going to use it, you're going to be betting on it all the way, really. you can use it if someone's just standing stupidly beneath you expecting an airdodge (come on, we're better than that). it'll get a few hits and cause a push once bowser lands.
a lot of players like to bombard bowser or rush him down when he's open, so instinctively a lot of players won't wait to see what you'll do next--they'll just attack you thinking that they can punish bowser. this is a good chance to throw out a random move of your choice (jab, ftilt, dtilt, fsmash) and watch you work your magic. this will only work a couple of times but it's a nice mindgame and the disjoint on fsmash will cause most attacks to whiff (especially at the distance that DAir creates) and you can retaliate with your own attack which will probably kill.
of course, this is a generalization and this varies from player to player, amd from match to match. it's on you to adapt and become a better Brawl player--this in turn will affect how you play bowser and as you learn more about the character, will make you a much more threatening Bowser player in general. being a brawl player and a bowser player are two different things, but they are still heavily related.
here's a quirky little fact:
At the center of it, Bowser's DAir is also a semi-spike, similar to Kirby's DAir. At the center of it, there is a hitbox that is designed to send the opponent straight down, while everything else just does stuff. if you do a short hop down air and get it right, you can still make it back and score an edgeguard depending on who you're fighting.
That's some OD **** though, I wouldn't recommend it unless you're ****ing around or something like that.
You can ALSO do a walkoff DAir and pretty much guarantee a spike and STILL make it back, but that's even more ridiculous than what I mentioned previously, and probably more difficult. Doing a walk-off move is the same thing as how Ikes, Falcos, Marths and other characters do walk-off DAirs.
