The icy rock is the method for evolving Eevee into Glaceon. You bring an Eevee, level it up around the icy rock, and it will evolve.
It evolves Eevee into Glaceon when you level it up near the rock.
Eevee, when leveled up in the presence of the Icy Rock, evolves into Glaceon. That's the only purpose the rock serves.