A waterproof mattress protector can absolutely make your mattress last longer. Mattress protectors can defend your mattress against everything from spills to odors to bed bugs. Impermeable protectors don't allow liquid through to your mattress.
That does a lot more than prevent a spilled glass of water from getting into your mattress foam. It also keeps out sweat, body fluids, and even urine if you have a child who still wets the bed. Keeping liquid out of the mattress helps to prevent odors from ruining it and requiring you to replace it. Controlling for liquids also keeps out another mattress ruiner: microbes.
Mattress protectors don’t just protect your mattress from moisture buildup that can lead to mold and mildew. They can also provide a physical barrier from pests like dust mites and bed bugs that can cause allergy symptoms and other issues.