At the end, he unleashes a huge wave of psychic energy that crashes Giovanni's helicopter and teleports an entire lake and its surrounding ecosystem UNDERGROUND, inhabitants included. This is after having been brought back from the dead when thrown into a Healing Spring.
The speech he makes just before is awesome too:
"You will not defile this place. It does not belong to you - any more than I do!"
And guess who gets yet another one risking his life to destroy the machines holding him prisoner? You got it, Mewtwo.
Afterwards, Meowth of all Pokémon gets one by making a speech to Mewtwo on why he should NOT erase everyone's memories again, because the cloned Pokémon have as much right to remember their pasts and heritages as any other Pokémon. Doubles as a Crowning Moment of Heartwarming.
Meowth: It's only natural to wonder about who and where you came from and someday these little ones are gonna want some answers. Only there wont be any, not if you make everybody forget! So what if their parents were clones? Their all living creatures. They deserve to live, and they deserve to know about themselves as much as you, or me, or anybody does!
Brock's absolutely brilliant tactical move where he saves his own life and probably the lives of his friends with nothing except a well-placed frying pan. Seen here.
...I don't get it...
OP is making a joke. If you want a legit reason, then here: Brock is the group chef, if he gets too sick to cook, the entire party, all 22 of them (3 trainers, 6 Party Pokemon per trainer, and Togepi), would starve to death.
Bug Pokémon everywhere got theirs in when, after our heroes have been captured along with all the other clone Pokémon by Team Rocket, the local Bug Pokémon show up and take down an entire Team Rocket Base. That's right, Bug Pokémon got a CMOA.