I have stayed as a youth hostel with a waterproof mattress before. It was more a thin piece of really shiny, really slippery plastic. The sheets just slipped off of it and in the end I gave up and just slept directly on it. Was as uncomfortable as it sounds.
As to your question, my bed has a "mattress protector" on it. What that does I have no idea. Might be waterproof to an extent, I have no idea. Just gets washed occasionally, guess it's because mattresses can't get cleaned so would get dirty over time. I don't know, really.
And jk1, they go underneath the sheets and sweat probably accumulates in them as opposed to the mattress which cannot be cleaned. So they need washing occasionally but it does save the mattress from all the sweat and stuff.
Hi! A mattress cover also protects the mattress and gives a little more cushion to it. I haven't used a plastic cover, and it sounds like I haven't missed anything.