I'd say convergence is the biggest plus of developments like this. USB 3.0 is sufficient for transfers between a host and a single client, but doubling bandwidth opens the gate for multi-purpose docks and other solutions without sacrificing bandwidth for the devices connected to them. I'd much rather be able to connect all my non-permanently attached devices to my monitor or a desktop hub than various ports on my PC without sacrificing speed. Also, for devices like the Surface Pro, this allows much better docking opportunities through a single USB port. Oh, and charging, of course. Come home, connect a single cable to your laptop/tablet, have a full desktop setup ready to use? Yes please.
Tl;dr: 3.1 is future proofing USB for mobile/thin and light devices.