It used to be that you’d have two computers because a laptop wasn’t powerful enough to be your main computer. But then, as the power of laptops grew and the price of them shrank, we began using laptops as our only machines — solving that horrible bother of with keeping the two machines in sync. But now, as it’s getting easier to keep things in sync (if our stuff isn’t all in the Cloud already), it seems there is this a migration back towards having a desktop and a portable.
Also worth noting is that the definition of “desktop” and “portable” is changing.