Secondary fermenter is a misnomer- it's actually a clearing tank, or called a "bright tank" in breweries. I make wine, too, and in wine there really IS a secondary fermenter so I think we brewers just adopted the terminology.
The purpose of the clearing tank is just that- to allow the beer to clear and some of the yeast to fall out of suspension. You should move it there AFTER the fermentation is over. Sometimes it will drop a couple of points, but you can't count on it.
Beer can stay in the primary for a long time before the yeast starts to autolyze and cause off flavors, so a clearing tank isn't necessary. Still, I almost always use one. You should see the "gunk" that I have in the bottom of my carboy after I rack from the clearing tank. That is stuff that would have ended up in my bottles, if I hadn't had used the clearing tank.