1. If you want to use autosense for inputs (for example, so that the Wiim Pro switches to the optical input when you switch on the TV) then the Wiim Pro has to be in standby.
2. If you want devices connected to outputs (DACs or Amps) to use their autosense to switch off then the Wiim Pro has to go into standby. This does not happen for the Wiim Pro optical inputs at the moment.
3. Presumably, the Wiim Pro only updates automatically when in standby otherwise it would interrupt listening.
4. If you want to reduce your annual electricity bill by 2p then use standby.