Its to accomodate the parking of aircraft.
It's a common trait throughout the UK, you may not like it, but the revenue earned from the parking fees vastly outweighs any income which could possibly recieved from smaller a/c landing upon it.
To put it in very basic terms, to keep a runway open and licensed by the CAA costs many £1000s each year, if the income from landing fees on that runway doesn't exceed the running costs it's a no brainer
As I've often said with anyone who visits these forums, if you don't like it, try running it yourself on a commercial basis if you dare, (and I doubt that you'd have the balls to do so or could afford it), and then you'll find out what the real world is all about.