Possibly to absorb heat from the sun during the daytime, then dissipate warmth for the soil throughout the night.

EG

Agreed with EG.

Dan

I imagine they double as mulch too.

I would be worried they'd provide breeding grounds for unwanted pests. They sure look nice though!

When I plant my peppers, I have some black rocks I'm going to put near them for the heat (bigger rocks though). We have such cool nights, even in summer, I think they might help.

Here is a link that might be useful: Tales of a Transplanted Gardener

Actually, light colored stones, reflect light, reducing passive solar gain (because of their color and reduced surface area - compared to the [dark] surface area of the exposed soil) and making the soil cooler .... but whoever put them there doesn't necessarily know that. ;o)

White crushed granite is often used on top of the soil and under small containerized nursery trees (especially conifers) under cover to reduce soil temperatures and reflect additional light onto the underside of the leaves and increase photosynthesizing ability (also to prevent the lower branches from die-back due to shade-out).

The large stones could also be there simply to help stabilize roots in what appears to be a lofty soil and help prevent the tall plant from toppling in the wind. ;o)

Al