You paid a considerable sum for your mattress. So what protects it? In most homes the answer is: nothing. The mattress protector may be the least noticed and hardest working piece of home textiles.
A person loses a considerable amount of moisture through sweat during the night. Some of it passes into the mattress. Over the years, the moisture, skin flakes and sweat that build up inside a mattress turn into stains and an ideal home for dust mites. You cannot wash a mattress; but you can wash a mattress protector. That is the whole point.
Waterproof protectors used to mean rustling plastic covers that made you sweat. Today good protectors use a very thin TPU membrane. It keeps liquids out but lets some water vapour through. The cotton terry surface on top keeps the part that touches the skin comfortable. In homes with children or pets, and in elderly care, it is not a luxury but a plain necessity.