Shoulders
While swimming, our shoulders have to go through many different motions. For freestyle there is mainly an internal shoulder rotation taking place, for which not only the rotator cuffs, but also the pectoralis major (main chest muscle) and lats (middle back muscle) contribute to the forward movement.
All the forward pulling and internal rotation results in tight internal rotator cuffs. As muscles need to be balanced by their counter parts, the weaker external rotator cuff has to compensate by allowing better flexibility and mobility. Think of the forward shoulder haunch of some swimmers, or in fact most people that tend to spend many hours behind a desk.
The aim of the inward shoulder rotation should therefore not be strength, as for the external rotators, but flexibility.