Car washes recycle water. Assuming someone is maintaining the filtration system, there may be less grit in the water but the dissolved crap will still be there.
A pressure wash will drive water into recesses where it maybe shouldn't become lodged.
If you live where it's warm and dry this might not be a problem. If it's cool and damp; the water you injected a few months ago may stay in those crevices for months longer.
This is the stuff I heard; makes sense to me.