UC useful to add more insulation + keeps foam dry
In addition to it's wind-blocking duties, the UC will also keep the foam pad dry in a rainstorm if you are using the standard tarp.
I've also employed it to add extensive additional multi-purpose under-insulation: down parkas, fleece jackets, etc. When winter camping in Minnesota I'd wear my down parka in camp at night, then put it under my lower back in the UC before retiring.
The price seems high, but its really just like buying a small tarp, but with all the added elastics and extra stitching for the bottom entry slit.