But before I start, two caveats:
- the outcome of a job candidate is not determined entirely by the placement efforts of individuals and departments--there are both the individual candidate's qualities and performance in play and the politics of the hiring department.
- there are departments and individual advisers who suck at this.
OK, the department logic is pretty simple: the reputation of a department hangs on a few things, with the placement of their grad students being a key ingredient. If departments do not place their students, the word does get out to a degree, making it harder (although not impossible) to attract good graduate students. It might also make it harder to hire faculty since people tend to prefer to be at places with upward trajectories. It is also something that Deans care about, as they prefer to have departments that do well in the rankings than departments that do poorly. They can direct resources to and away from departments, so their concern about rankings is important.