When I use down smash, it's usually for ledge traps, or ledge wake-up punishes but it can also be used against rolls. While it does have some similarities to Up b, it helps to keep in mind that the hitbox is better unless you're looking at the last hit of fortress. It has the same range as Brawl Down smash pretty much, but instead of the shell being then entire hitbox, there are significant disjoints on either side which can protect Bowser, and make the attack pretty much impossible to challenge without a proper disjoint or from certain angles.
When using it in ledge traps, it's normally following a ledge trump because it hits the ledge and will punish them for grabbing it, some characters also can't jump over it and will land in it. It can also win against ledge get-up options depending on the character, for instance, if spaced properly, it should beat any standard ledge option that Marth can use, and it'll also beat ledge hop attacks that attempt to jump directly onto the stage. You could also try to go for frame 1 ledge shenanigans on certain characters, which can lead to a stage spike if they get pulled in just right.
Despite it's drawbacks, I still like Dsmash, and want to find practical uses for it as well, so I'll keep looking.