The main benefit of a waterproof spray is that it protects your outdoor gear from water damage. It forms a fine coating on the surface that makes water form into beads that roll off instead of soaking in. Wet fabric loosens and deteriorates over time and this is a great way to prevent this process. As a bonus, this prevents mold from forming too.
Another benefit is stain protection. The coating will protect the fabric from strong-colored foods and drinks, making it much easier to clean. Finally, some products provide excellent protection from UV rays too. Outdoor equipment like patio furniture is constantly exposed to the sun, and the best waterproof sprays can prevent color fading, yellowing, or deterioration.