As I am teaching CS 17, a freshman course with a large enrollment, this Fall I have a whole army of TAs: more than a dozen, plus two head TAs. Between them, they manage the course and take care of pretty much everything other than lecturing: organization, labs, homework, projects, grading, office hours, etc.
What is the role of the professor in this situation?
One role is to provide a reassuring presence vested with some status that comes with the position.
Another role is to take responsibility if anything goes awry.
Another role is to have decisional power for the final grades at the end of the semester.
Another role is to update the course content according to my vast (ahem) knowledge of computer science at large.
Another role is to help resolve difficulties that may come up, by providing the superior (ahem) counsel that comes from experience - all the TAs are themselves undergraduates, so this is an example of students teaching students.
Without the TAs, even if I spent 100 percent of my time working on the course, it would be simply impossible to teach the course in the way in which it is now done. But without me, the TAs could get organized to add lecturing to their already long list of tasks, and the course would go fine. However it would be difficult for them to assign final grades, and as the course evolves from year to year, I think that it would be difficult to maintain its coherence. In some ways, I am the head that officially represents the course, and one of my tasks is to provide both the continuity and the reforms that should take place from year to year, as we analyze what went well or not so well and brainstorm for modifications for the following year.
I am the Pope of CS17. The head TAs are the Roman Curia, the TAs are the dedicated priests, and the students are the Faithful.