Rigid foam boards stacked near recently excavated foundations: what's it for?

On my daily walks with the kids through the new development one neighborhood over, we pass houses in all various stages of construction. Next to the ones that are only a freshly dug hole in the ground, there is usually a bundle of rebar, a pile of gravel, and a stack of 8x4 rigid foam boards. The stack is never more than maybe 6 or 8 boards thick.

I try to give educated guesses to my son about what they're going to do with all the materials I can see, but I'm genuinely puzzled about what...

Rigid foam boards stacked near recently excavated foundations: what's it for?