Are you talking about down-b, or just b? It's kind of ambiguous to me from your first post.
Both of those moves are used very often in competitive play though. Down-B (Thunder) is used to knock opponents off the top of the stage after an upsmash (or sometimes upair). Example video:
https://www.youtube.com/watch?v=pgocyJ5Kb-o
Neutral-B (Jolt) is used mostly to cover some recovery options of your opponents, especially against Fox and Falco who have predictable side-b recoveries, and slow enough up-b recoveries that you can react to them if the jolt misses. It's also good to throw out as you're recovering to distract your opponent and prevent them from positioning optimally to cover some of your recovery options. Example video:
https://www.youtube.com/watch?v=XPL9RK82NEc
I've posted both videos without timestamps cause I think they're worth watching all the way through. Really really good sets to study, but look for the things I've mentioned!
As a side note, I find jolt to be useful in low level play when you want to stop somebody from ledge stalling. Axe knows better than I do how to deal with ledge stalling, so I trust that there's a better way since I never see him do it, but if I see someone just regrabbing the ledge I'll shoot some jolts at them to help convince them to get back on the stage.