I use them all the time for marks on the walls and have never noticed matte spots from them. Maybe if you scrub a lot with them? 

I mostly use them on messes the kids make. They’re great for getting marker off surfaces. It even works on sharpie. Also good for stains like if the kids spilled food and it didn’t get cleaned up right away.

I buy the big packs of 18 from costco and it lasts me about a year.

I never use them either. Don't even buy them. I have absolutely no idea what people use them for when they have to buy them constantly? On surfaces that can be cleaned with water, they're almost never necessary. On surfaces that are delicate, like walls, they just matte the paint and sand down wallpaper.

It's just melamine foam. I bought a couple of Great Value brand at Walmart just to see what they could do. They're still sitting in the back of a cupboard somewhere. Never actually found a use for them.

Years ago Reddit taught me that Amazon’s magic eraser sponges are just as good and a fraction of the price.