A straight, unswept wing experiences high drag as it approaches the speed of sound, due to the progressive buildup of sonic shockwaves. Sweeping the wing at an angle, whether backwards or forwards, delays their onset and reduces their overall drag. However it also reduces the overall span of a given wing, leading to poor cruise efficiency and high takeoff and landing speeds.
A fixed wing must be a compromise between these two requirements. Varying the sweep in flight allows it to be optimised for each phase of flight, offering a smaller aircraft with higher performance. However it has disadvantages which must be allowed for. As the wing sweeps its centre of lift moves with it. Some mechanism, such as a sliding wing root or larger tail stabiliser, must be incorporated to trim out the changes and maintain level flight. The added weight of the sweep and trim mechanisms eat into the performance gains, while their complexity adds to cost and maintenance.
By moving the wing pivots outboard and only sweeping part of the wing, the trim changes are reduced, but so too is the variation in span and accompanying operational flexibility.
It is not necessary to sweep the port and starboard wings in the same sense - one can be swept back and the other forward, as in the oblique wing.
Varying the sweep asymmetrically by small amounts was also fundamental to the principle of the wing controlled aerodyne.