Applying a deck sealer/stain stain will greatly extend the life of a wood deck. The finish prevents the wood from going through wetting and drying cycles. If left untreated, precipitation will enter the wood and expand. Once the lumber begins to dry, it contracts. This expansion and contraction leads to the degrading of wood. Sealing will prevent water from entering and thereby greatly reduce the wetting and drying cycles.
A brand new deck will need time to dry (2 to 4 months after construction). A good indicator of adequate drying is when the deck has a tan color to it. Most decks are built using chemical/water pressure treated yellow pine. When the moisture from the treatment evaporates you have warping, cupping and splitting, which is notorious in treated yellow pine. No wood is immune to this, but a commercial grade oil based stain and sealer is your best defense.
Old/older deck needs to chemically stripped, pressure washed and wood brightened so that the wood is properly prepared for a commercial grade stain and sealer.