Another reason to retire is that you don't have to clear the dinner plates if you don't want to. Leave them on the table and come back to them once your guests leave. Even if you do clear the plates, the table might be scattered with food or littered with empty serving dishes. You won't have to spend any time cleaning up before you serve dessert.
I find moving somewhere else facilitates new conversation (it can also give you pause to change the conversation if you've ventured into, say, a heated political debate that you'd like to curb). If you are into games (we are in our house), you can get things started over dessert. And putting the cake, pie, or cookie plate out on the coffee table lets guests serve themselves easily.
One thing I should note: Moving to a more relaxed, comfortable room for dessert means your guests may stick around longer than you planned. When you serve dessert at the table, then finally getting up from the table is a signal to get going. If your friends are hanging out in arm chairs, they may stay put for hours. Not that that's a problem; I'm just giving you a head's up.
Of course, those of you with a porch or patio may want to retire to the outdoors, even if you ate in your dining room. Or vice versa—eat outside, come in for dessert when the sun goes down. What do you do?