It really is a matter of convenience. Linus's work is obviously mostly with computers and servers, where you often work in tighter spaces; not having to reposition your whole hand (and/or pull the screwdriver out of the screw and reposition it because of a lack of space) is a nice convenience. Plus, most computer screws are not really tightened very much, so you'll be done with a few flicks of your wrist. He also explicitly mentions he uses the ratchet by turning only the axle and keeping the handle steady in his hand for a lot of the screws.