I see two important reasons for doing isolation movements, and neither have anything to do with building mass.
#1. Some isolation movements can help bring up your bigger lifts in the form of assistance exercises. An example of this could be doing curls to help control the negative portion of the bench press or leg curls to help the squat.
#2. I believe some isolation movements are smart in order to work the joints in their natural movements and create a healthy balance. An example of this would be doing leg curls again. Most of the posterior movements we do in a compound-only routine work the posterior with a straight leg. I believe it's important for health reasons to also work them in the bending motion. Other examples are L-fly movements to work the rotator cuffs, and abdominal exercises to balance your back strength.