I suspect 'at work' might include schlepping down to the lecture hall and plugging into the projector now and again. Or popping over to the Library to make notes. Or sitting in a faculty meeting to take the minutes.
To directly answer your point: portability within the building/location.
Most schools have dedicated setups for classroom presentations. I think jgm is at stanford, I imagine stanford has the cash and competency for a decent classroom setup.
In addition to portability for when you do need/want it, like others mentioned, just plain saving space. If I didn't still want to play the occasional game of Civ V with high graphics settings, I'd love to ditch the tower that sits next to my desk at home.
