Why Brock give me a TM case only rock tomb?

It seems you're asking about a situation in the Pokémon games where Brock, the first Gym Leader in Kanto, gives you a TM containing the move Rock Tomb. This is actually not how the games work.

Here's why:

* Brock's TM: Brock doesn't give you a TM at all in the original Pokémon games. He gives you a TM Case, which is an item that allows you to carry more TMs.

* Rock Tomb: In the original games, Rock Tomb was not a TM move you could learn. It was a move that some Pokémon could learn by leveling up.

Possible Scenarios:

* Different Game Version: If you're playing a different version of the Pokémon games (e.g., a fan-made game or a hack), it's possible that Brock gives you a Rock Tomb TM.

* Misremembering: It's possible you are misremembering the experience. Brock does give you a TM case, but not a TM itself.

If you could clarify what game you're playing, I might be able to give you a more specific answer.