After Escaping from the clone laboratory on Cinnebar Island, Mewtwo met Giovanni where they entered into an agreement where Giovanni would focus Mewtwo's powers and Mewtwo would help Giovanni's goals.
Giovanni provided Mewtwo with a suit which would help Mewtwo focus his powers and it was given the chance to show his power when battling for Giovanni at his Pokémon Gym in Viridian City.
Mewtwo destroyed Team Rocket's HQ and shed the robotic suit after it discovered that Giovanni was using him for his powers