The building description for a Bus Depot says that it "...automatically sends out buses for your bus lines". From what I've seen though, once a bus arrives at a newly-created line, it will stay on it forever. (Bus drivers in Cities: Skylines do not appear to require sleep, nor do the buses require maintenance.)
It seems to me that there isn't any real purpose in putting a bus depot anywhere in particular, as long as you plop one down somewhere on the map to unlock the Bus Line tool. Is this accurate, or am I missing some key aspect of the way buses work?