What is the purpose of the Variable sweep on some Fighter Planes and Bombers?

The principal reason is to increase the speed range that the aircraft can operate in. In the forward swept position, most designs expose greater wing surface to the slipstream, improving lift and handling at low airspeeds, such as takeoff and landing. At speed, the wings are swept back (and partially and partially retracted within the fuselage on many designs) to reduce drag at trans sonic and super sonic speeds. Additionally, sweeping the wings back moves the "cone" where the tip vorticies come together at supersonic speed to a point well behind the tail to prevent the resulting shock wave (that also causes the sonic boom at supersonic speeds) from damaging the tail embly with catastrophic results.