At the shoulder, internal rotation turns the palm toward the body and external rotation turns it away. The internal rotators (lats, pecs, subscapularis) are trained by nearly every press and pull; the external rotators (infraspinatus, teres minor) are trained by almost nothing unless you program them.
That imbalance is why face pulls and band external rotations sit in so many upper-body sessions. Two sets of 15 at a weight the client can control through the full range, three times a week, is a maintenance dose. Loading it heavily defeats the point, since the muscles are small and the job is control.
At the hip, external rotation is what opens the knees in a squat. A client who cannot hold their knees out under load usually needs the cue and some direct glute medius work, not a wider stance.