Eurotunnel offer different fare rates [depending on how long your trip is](http://www.eurotunnel.com/uk/tickets/types/day-and-overnight/). For example, their cheapest is the day trip / overnight rate. Next is the short trip, and finally is the "long" stay.

The question is, why do they offer this? What possible difference could it make to Eurotunnel to charge me less if I come back sooner?