I've just started using a secondary. I'm not sure what the advantages are exactly, but here are some observations.
I hear using a secondary is primary for clarity. When I pull my beer into the secondary I leave a huge cake of hops, yeast and other solids behind. I can tell that these solids havce no chance of getting mixed up in my beer again... since they were left behind.
Also, 15 minutes after I moved my beer to secondary my airlock started bubbling again... somehow the move made the yeast start acting again... not sure if this is normal or not, but was a cool result.