Towards entering Route 9 , a few studs away from the entrancethere is a patch of tall grass with a gray rock containing some
sort of moss on its head.
The rock seems to be almost fenced off, although it is not.
To get to it, you'll have to go to the bridge that leads to Route 10, but instead of going across, turn left and go through the trees, and you should be able to access the Moss Rock.
The Moss Rock is used for evolving an Eevee to a Leafeon.
To do so, you must 'train' your Eevee in the patch of grass surronding the rock, and once it has levelled up at least once (if it is not holding an Everstone) it shall turn into a Leafeon.
And there you have it. The purpose of that strange rock revealed.