I’m guessing it was to protect the speakers…but obviously it will affect the sound in a big way.
I’ve mentioned before that I use to run 3 grill cloths on an early JCM800 slant cabinet. It had large checkers, then basketweave, then black. It was a tight fit getting the baffle in, but it sounded great with H75’s. Keep in mind I used a slightly modded JCM 800 2204 with it. Many times I was running the amp on 6. It was stupid loud.