How important is it for a professor to direct a thesis?
Advising a bachelor or master's thesis is just part of my teaching obligation. Most of the time it is an enjoyable obligation: now I get to accompany someone on their first steps in doing independent research. However, that is pretty much all I get out of it. Advising a PhD thesis is a bit more prestigious, but the added value drops quickly. However, with a PhD project the advisor may get one or two co-authored papers out of it, and in some disciplines the PhD students are an important source of cheap labor.