Gary Becker actually came up with a very elegant theory to explain this phenomenon: an individual consumer's desire to eat at a given restaurant is often driven by the observation that other consumers are eating at the same restaurant. Long lines give restaurants the social proof they need to maintain profit-maximizing levels of demand.

The original paper: http://www.unc.edu/~fbaum/teaching/PLSC541_Fall06/Becker%20J...