The purpose of this lane is simple, to overtake, nothing more and nothing less. If you are in the center lane doing 70 mph and a lorry pulls into your lane doing 65 mph because he is overtaking another lorry traveling at 50 mph, you have the outside lane to move to so you may maintain your speed at 70 mph until it is safe to move back to the center lane. The outside lane is NOT just another lane to drive in, and it is very important to leave this lane free specifically for overtaking. It is actually illegal to Undertake a car (that is to pass a vehicle from the left hand side) so by driving slowly in the outside lane, you would be forcing other drivers to break the law, should they wish to pass you.