Once you are airside and trapped in the terminal for several hours, watching the planes would be one of the best ways of killing time, but often until you get to the gate, you can't see anything at all of the airport from the terminal.
Certainly at Luton or Heathrow, until you get to the gate, you could confuse it for a railway station.
A bar / restaurant with an outside viewing area would do good business I bet.
Also alert spotters/passengers could help keep an eye on things, making sure engine cowlings were done up, that sort of thing...