Rubber bellows help to minimise the entry of moisture into the brake fluid reservoir (a minimise loss in performance of the brake fluid), the entry of dirt and other particles is also reduced. Bellows are particularly useful for endurance racing applications. Please note that using this below does reduce the capacity of the reservoir.
These are the small bellows which are suitable for is with CP4709 reservoirs (except for the CP4709-12, -13, -16 & -19 which are to small to fit bellows).