[OverTemp]

Simple answer... increase the Critical Mach number.

Rember Bernouli? The flow over the top is accelerated because it has farther to go around the airfoil. This increases dynamic pressure (the pressure cause by air movement). Since total pressure cannot change, static pressure (pressure caused by air sitting there) must decrease. This decrease in static air pressure on top of the wing is lift, or a major component of the total lift being produced.

Since the air is being accelerated over the top of the wing it's velocity is faster that the static air around it. At some aircraft speed below the speed of sound (usually Mach .75 or above, but I've seen some lower) the speed of the air relative to high points of the wing goes super-sonic. It's called local sonic flow (if I remember correctly). This creates a normal shockwave which creates a number of problems, including excessive drag, loss of lift, movement of the center of lift, etc... This magic speed depends on the airfoil and aircraft shape and it called the Critical Mach number.

By sweeping a wing we send the air over the airfoil at an angle... hard to picture, but some of the air is now moving spanwise along the airfoil instead of directly chordwise along the airfoil. This has the effect of extending the length of the chord line and reducing the harshness of the angles the air must travel over... this lowers the local air velocities. Thus, the airplane can now fly faster before reaching the new, lower, Critical Mach Number.

Swept wings have draw backs... they want to dutch roll (yawing in 1 direction while rolling in another... really bad at high altitudes) so now we have yaw dampers installed in airplanes. The want to stall tip first, and geometry changes and devices are needed to keep aileron effectiveness at low speeds. If the root stalls first the plane wants to tuck over into a nose dive... the balance of lift vs. AOA along the wing must be very carefully balanced. There are numerous other disadvantages but the increase in speed and efficiency is worth it.

Aero is simply a balance of compromises... nothing is free.

I fly heavy full scale... what I've quoted here is all regurgitation from courses etc... over the years and practical experience... I'm sure some of it is a little off though.