What is the rock covered in moss for in the eterna forest?

that rock is for getting leafon if you level up an eevee there it will evolve into a leafon

The rock in Eterna Forest is nothing until u beat the league

It will then let u evoleve EEVEE

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At least in Platinum you can evolve Eevee into Leafeon as soon as you get Eevee. I just got it level 21 (1+ from level 20, the level the girl gives Eevee to you)