It also helps protect the liner from stuff poking through from underneath.No but it's just me in there and any dirt or sand that I get in there I can just vacuum out before I put down the liner.
I've decided that I'm going to pack dry concrete where the groundwater is flowing through the worst at the wall bases, widen the base of the wall there, smooth everything out with sand, and then put scrap carpet on top of all of that to try to have a semi-firm base to walk on when I'm spreading out the liner. I think that will work.