Generally speaking, when is it necessary to install a sub-panel for providing electricity to an outside shed/office?
As far as I understand it, since the 2002 version of National Electrical Code, only a single branch circuit or feeder is allowed to supply a building (225.30). Though it's possible that your local Authority Having Jurisdiction (AHJ) allows it under one of the exceptions, or there are local amendments. 

There's also the consideration that the NEC requires a disconnect at the building (225.31), so installing a panel may make more sense.

It's possible that the Electrician is going to install a single 20 ampere multi-wire branch circuit, which would be allowed under 225.30. Though he's still going to have to install a disconnect at the building.



To answer your question. The first reason to install a second panel, is to allow for future expansion. Though this doesn't seem to be important in your situation. 

The second reason is simplicity. You can only have one branch circuit or feeder, and you need a disconnect. It just may be easier to install the second panel.