Let's say you work in a private college and most students have a right to personal tutoring. They can follow pre-recorded audio courses at their pace and have an assigned tutor which is available to them at any pre-agreed time.
Without sessions, you would have one course sharing several tutors and several students, and each tutor would share the “space” with others and would see the results of students he isn't in charge of. Worst, he could even modify the results of a students he isn't in charge of...
With the sessions, simply define one new session per student. Register the courses he has access to and his own coach to the session. These two will now be able to work in an isolated virtual space.