I think the biggest benefit of a zippered waterproof mattress cover is that it protects the mattress from the everyday moisture and spills that are difficult to avoid. Things like sweat, drinks, or accidental liquid spills can gradually affect a mattress if there is no protective layer. A washable cover also makes regular cleaning much simpler, since you can remove it and wash it instead of trying to clean the mattress itself. For daily use, choosing a breathable material and the correct size seems just as important as the waterproof protection, because it helps maintain both cleanliness and sleeping comfort.