Eurotunnel offer different fare rates depending on how long your trip is. 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?
Price discrimination. Eurotunnel has figured out that there are different markets for different types of stays, who will pay different amounts, so they set prices accordingly.
See also: Why are plane tickets more expensive if they don't include a weekend?
And Wikipedia goes into this in detail: Price discrimination
Traditionally it's so that people can drive across for the day to load up on cheap booze in Calais, either for themselves or (illegally) for resale. That's been a source of business for the tunnel and ferry operators for many years.
(People on day shopping trips are highly price sensitive as they're only making the trip in order to make money on the tax differential between France and the UK).
