Waterproof dog beds save you from replacing foam cores ruined by accidents, drool, or wet dogs coming in from the rain. The key distinction: "water-resistant" means it repels light splashes; "waterproof" means the inner liner fully protects the foam even if the cover gets soaked through.
Look for beds with both a waterproof inner liner AND a machine-washable outer cover. The best setups let you unzip the cover, toss it in the wash, and wipe down the liner. Some beds use fully encased waterproof foam, which is the most bulletproof option for incontinent seniors.
Fair warning: fully waterproof covers can trap heat and feel plasticky. Better beds use a breathable waterproof membrane (similar to Gore-Tex) that blocks liquids without turning the bed into a sauna. If your dog runs hot, look specifically for this feature.