U-Cups maintain sealing contact to the piston rod when the equipment is stationary (static, no reciprocating motion of rod) and in use (dynamic, reciprocating rod), whereas the tolerance for the rod diameter d is determined by the rod seal. Without a U-Cups, the retracting piston rod could transport contaminants into the cylinder.

The outside static sealing of the U-Cups within the housing is also important to avoid moisture or particles from entering around the outside of the wiper seal.