Side B is used as a recovery option on some stages. You can do it repeatedly to climb walls, the technique is called Wallbombing.
In some situations it is your only hope of surviving and you should know how to do it if you play the character at a high level. Some characters don't have great options at stopping it.
I made a vid on how to perform the technique:
https://www.youtube.com/watch?v=beK7myWnvSU
Outside of that you can use it for extremely situational combos and as a shield breaker.
https://www.youtube.com/watch?v=8BBlLUFpkGs
https://www.youtube.com/watch?v=_SN-8e8jjqg
in an actual game:
https://www.youtube.com/watch?v=oOyn3q8GXIs
Throwing out randomly in a game has yet to be proven to be effective.
As for Toad, it is also very rarely seen. There are some really niche stuff it can do:
- Turn around in the air (reverse B)
- Stall in the air (time out strats or waiting for teammates to respawn in doubles)
- Recovering back to the stage and predicting the enemy to attack
- Breaking tumble so you can then float (works for if you have lost your double jump or if you need to conserve your DJ like when you need to Wallbomb on Fountain of Dreams
- Calling out very obvious projectiles esp vs Falco, Peach and Samus.
A lot of these uses either have alternatives that are just as effective though and generally the overall impact is minor. The move is slow to come out, inconsistent (the spores are random) and it doesn't fully protect Peach so you can just get hit anyway depending on the attack. Its also so laggy that sometimes the opponent can get hit and still recover in time and hit you before the move has ended.
If you are new to the game then you can basically ignore these moves altogether, if you are a bit more advanced then you should know how to Wallbomb.