I think it serves a few purposes. One is for heat absorption, especially if the squirrel is young and the rock is warm. I learned this when we found a young squirrel splayed out like that on our driveway in the sun. It was a hot day and I thought he (or she) would burn on the driveway, but as it turned out, he was looking for the heat.
Even when they're old enough to live on their own, for a period of weeks the mother still spends some time with them and shares her body heat. This one had likely gotten separated from her and was looking for his own warm spot, which he found.
Fearing for his life, we brought him to the Wildlife Clinic of the Cummings Veterinary Medical Center, a world-renowned leader in the field of wildlife research and care. We left him overnight so they could run tests, but he was found to be totally healthy. We brought him back and he stayed in our yard for two years, after which we moved away. I hope he's still there!