Even though your deck was most likely built with pressure-treated wood, it will still need some sort of sealant to make it waterproof. Not only will this help protect it against sun damage and the elements, but it also keeps it from absorbing the party beverages that will inevitably be spilled on it, like beer and fruit punch. Even if your deck has been sealed in the past, it may need another coat. Deck products are only meant to last a couple of years at most before they need to be reapplied. The ideal time to seal is right after you give it a good cleaning.