At the end of the lease, you have several options.  Firstly, you can sell your car and pay off the residual which will allow you to lease another car.  Also, if you sell your existing car for more than the residual you keep the tax free profit.  The second option is to extend your lease – allowing you to keep your existing car longer.
Lastly, you can pay off the residual and keep your existing car.  This would mostly be of benefit to those that are no longer employed or about to retire.