Just to be anal about it: It's not really a secondary fermenter. Secondary fermentation is just another stage of regular ol' fermentation and should happen in your primary before you consider racking to another vessel.
What most people think of as a secondary is really just a vessel to get the beer off the trub and yeast cake, which can allow for a clearer beer, or more conditioning time before bottling/kegging without any risk of off flavors from trub or yeast (which I'm led to understand would take quite a while to develop in most cases even without a secondary).
But like the others have said, in most cases it's unnecessary. I like to use a secondary because I think it does lead to clearer beer, if only because there's less sediment in the secondary when I rack to the bottling bucket. But I'm not sure there's any real point in it most of the time from a flavor standpoint.