I certainly get a chance to see my students as an observer, as someone who can help them even more when I come back to teach full time once the student-teacher's time has ended.
Doing so (without stepping in on occasion) is tough. On one hand, you don't want the misbehavior (or disrespect) to happen and/or continue, but on the other hand, you want the student teacher to know how to handle the situation.
What I usually do is role model as many different situations in the first few weeks of the experience for the student teacher, and then we discuss almost all of those situations that same day (knowing full well that it's going to happen to them soon enough).
Then, when they start to teach on a more regular basis, I step back and try to let them remember our discussion about the situations that happened to me while I was teaching. The last thing I would want to do is just "tell them the answer."
All in all, it is very beneficial to have someone come into your classroom, because ultimately, it helps our classroom (and me as a teacher) improve overall.