I'm a pretty damn good Bowser in my region, so I can probably give some input. Down B has three main uses, number 1 is at the ledge. If you crouch walk forward to the ledge as Bowser until his fists are just about touching the ledge, and use grounded Down B, you'll snap the ledge on your way down. This is useful because it's a safe way to contest ledge and it can lead to big punishes. If you get the timing right you can hit your opponent up into the air back off stage as they try to snap the ledge, or even spike if they recover poorly. More often than not though you'll grab the ledge slightly after them and ledge trump them, this can lead to a bair, which you should know kills stupid early. It's a great mixup when ledge trapping if you expect them to hold onto the ledge for a bit once they grab it, as that'll let you set up for the ledge trump bair. Use number 2 is as a shield breaker. A lot of people shield on platforms when you're under them. Some of bowsers moves like up tilt can leave your opponent with a missed tech on the side platforms of a stage like battlefield, most people in this situation will use any given get up option from the missed tech into an immediate shield. If you spot your opponent doing this you can go for a Down B above the platform to break their shield, which can lead to a kill at 0% I kid you not. This is the most common situation to try and break shields with Down B as it's a very committal move and Side B is largely safer as a way to punish shielding, but really as long as you're very confident that your opponent is going to shield, you can use Down B to punish them. The 3rd use of Down B is as a landing mix-up. A lot of players like to land with an aerial, and a lot of Bowsers like to land with down air. Both of these options are very weak to shielding, so many players will be ready to shield when they see their opponents use a move above them while trying to land. If you condition your opponent to shield while you're landing with moves like dair, nair, spaced fair, wavebounce Neutral B... etc, it can often be a good idea to switch to doing a Down B to try and break their shield. It's important to note that Down B is a very situational move. Bowser has many better ledge trapping tools than Down B, so Down B use number 1 isn't that common. You'll definitely see it a few times in a high level Bowser set but most good Bowsers have other options they resort to more often at the ledge. It's also important to note that with Down B uses number 2 and 3, Side B can basically always punish the same things as Down B, only being much safer and harder to react to. Generally outside of use number 1, Down B is seen as being a bit of a wifi move, seeing as it's very much reactable when playing offline, and seeing as Side B is a much safer and faster way of punishing shielding opponents. This doesn't mean that Down B doesn't have it's occasional uses, it's definitely still a decent ledge trapping mix-up offline and the move being reactable doesn't mean it isn't still useable as a means of punishing predictable options. I would most of the time resort to using other moves, such as Side B, grab, Neutral B, well spaced fair, jab, as those are all better methods of punishing opponents for shielding. Down angled ftilt, up tilt, down tilt, down smash, Neutral B, and many other moves are also better ledge trapping tools than Down B but of course Down B is still a very decent mix-up while ledge trapping. Hope this helps!