Beer ages faster at room temperature, so if you have a keg you want to age a bit it's best to keep it at room temperature. There are also some benefits to cold conditioning, including the clearing process. Many of my beers are APAs and IPAs which are pretty good young, so they will often spend three weeks in the fermenter and then into the kegerator when kegged. If it's not quite ready, though, I'll keep it at room temperature for a couple of weeks first.
I think that really depends on the style and OG of the beer. A big beer can keg condition and room temp for several months, whereas wheat beers and IPAs are best drank young. My last barleywine sat at room temp for 9+ months before I put it on tap.