Shock absorbers contain many moving parts to controls the oil flow that provides the damping action. After a certain period of time, the oil can degrade and lose its properties, affecting the consistency and efficiency of the damping action. It's pretty much the same thing that happens to your car when you need to change the oil.
Due to the amount of stress that shocks are put through, when regular used, this can cause some wear to many internal parts such as the valving shims, the o-rings and wear band. BSB Suspension recommends that you get your shocks serviced each year to ensure that all internal and external parts can perform to its fullest. Regular service and maintenance will also ensure that your products last longer.