We ordered the spray foam on our new 21, which we pick up in about 2 weeks! The spray foam (obviously) provides excellent insulation, and should help keep your feet from freezing in colder weather. In our now gone Casita, we'd put camping pads and carpet on the floor to make it more comfortable. Hopefully our spray foam will negate most of that extra effort.
Spray foam also help protect the underside of the trailer from damage. The Good and bad is, it does cover plumbing, tanks, etc,...Some people will argue that's bad, but if you need to, you can dig out the foam, do a repair, and re-foam it. I've had years of experience spray foaming houses, so I'm not afraid...don't be afraid.
Believe it or not, it also adds structural integrity and rigidity to whatever it sticks to, as well as quieting things down, providing thermal and sound insulation.
It's good stuff IMHO, though others will give contrasting opinions...which this forum is really, really good at...