Basically, the fabric of the strap constantly rubs up against the case back, and when dirt gets trapped in between, over time, that causes scratches and even marring on the case back. According to a dedicated article by British GQ, this kind of marring cannot be prevented by simply cleaning the strap. They say no matter what, the fabric will cause wear to the case back. Are they right?
Well, interchanging straps helps. Change em out before they get dirty enough. haha