1. Less prep. Obviously, it takes less time to buy somebody else's work and use it than do your own.
2. Pilfering it for ideas and inspiration to use in your game.
3. In the case of a campaign setting, you just really like it.
What I have to confess is that I don't really understand the value in playing somebody else's module straight from the page; it seems fundamentally inauthentic in the same way that cover versions or remakes of old films usually seem fundamentally inauthentic. Assuming a DM has a few scraps of sense and creativity about him, he can likely come up with material that will be as good as the stuff you find in a published module, with added spice coming from the fact that it is his own creation which he knows and loves and feels invested in.
People in the aforementioned Dwimmermount thread seem to be of the view that modules are useful because they can act as some sort of instructive tool: you run the published module and that teaches you how to create your own. I find myself wondering whether the effect might rather be to entrench standard practices, restrain innovation, and above all waste time: is trial and error creating your own material that much less effective than running through modules when it comes to learning how to DM?