It's an interesting topic you bring up (I did laugh at your pea comment too).

The cooking thing sounds particularly nonsensical in that you'd have to get a good fire going to heat the stones in the first place. So then you'd cook over the fire wouldn't you!

What sprang to my mind was the north american sweat lodges. I see with a quick google that that's certainly been suggested before. Could be a possibility? Everyone likes a nice bit of non-ordinary reality for a change, it's in every culture isn't it. But then why would there be so many of them found, would you not reuse the same site for ease? Or maybe you weren't allowed to reuse the same site (there's a good excuse).

Or maybe, it was for some industrial sort of thing where you had to have lots of hot water.

Or maybe they just liked a hot bath now and then and who can blame them, especially in the bleak midwinter.