Theoretically the pressure should empty the surface varicose veins. However there is no real evidence that this does anything useful and therefore the NICE guidelines specifically state that compression stockings for varicose veins do not help.
Despite this, it is common for GPs to recommend stockings because a decision has been made by the NHS not to refer patients to venous surgeons unless skin damage has already occurred. These skin changes may be permanent. However, since GPs are not supposed to refer patients for treatment some feel they have to do something, so offer stockings.