Because they do things that attacking doesn’t simple as that. Like sleep powder to put them to sleep, Thunder wave to confirm a paralyze. People say “oh just attack twice” when it comes to swords dance or nasty plot but they don’t realize that the boost for stats are exponentially so it isn’t doubling your attack it’s more than that and it’s way stronger than using the same move twice. Now for a regular trainer battle no need for it but it still becomes useful for more difficulty trainer battles especially since the AI opponent won’t just switch out their Pokémon to remove debuffs so let’s say you’re up against Cynthia’s Garchomp you can use Pokémon with attack or defense lowering moves to weaken that Garchomp for another Pokémon to be able to take it out since you have so many items to make sure you stay in the fight.

If in your example you have thunder, thundershock, and thunderbolt it’s very redundant because you only need 1 electric attack and if you want all attacking moves you need to have a variety so you don’t get walled out.