The intended use of a waterproof sheet is to protect a mattress from bed-wetting accidents. Potty training take practice, and accidents inevitably happen. Illnesses, nightmares or distractions can increase the chances of accidental bed wetting by your child.
Hotels across the world are being infested with bedbugs. Even if you do not travel much yourself, your neighbors might. If your children spend time with other children, they may unwittingly bring these pests into your home. Blankets and sheets can be washed, but an infested mattress is nearly impossible to completely clean. The use of a fully enclosed waterproof sheet can keep pests out of your mattress or even keep them inside, if it's already infested.
Children will be potty-trained eventually, but even well-trained pets can have accidents. Our loyal companions can also bring mud or fleas in from outside. Keeping your pet inside is one option, but it only takes one careless moment to allow your pet to escape. A waterproof sheet can protect your mattress from soggy dogs and cats who might be carrying fleas or ticks.
When you do replace your mattress, you may decide to keep the old one for a future use. The best way to protect a stored mattress from dust and mold is to keep it in a fully enclosed waterproof sheet. When you are ready to use the old mattress again, a waterproof sheet is much easier to clean than the actual mattress.