http://www.popularmechanics.com/flight/a19875/why-airplane-wings-angle-backwards-explainer/

By angling the wings of an aircraft toward the rear of the plane,
the airflow over the top of the wing is reduced. The plane can then fly faster
without generating shockwaves on the wings, giving us the commercial transonic
flight that we complain about today.

Thanks to Rodger Hoyt.