Another common example would be airport check-in. You have economy and first-class.

Often the first-class line is empty - do you let 'the rabble' check-in there and risk a first class passenger having to (gasp) wait a couple of minutes behind them; or make the long line of economy passengers stare at the unused desk and mutter about the airline.

(Though given that airports are really just a series of queues feeding each other the different in practice is pretty minimal.)