What Pattern said is true
But I'm not sure that this directly causes them to be curved.
I would also suggest (let the reader beware) that since there is a natural untwist moment that to counteract this the blade shape is offset from the blade stacking line. Where blades have low hub-to tip ratios (low by-pass) this offset is not easily seen. However in the giant machines the amount of twist is much greater and the stacking line tracks along a curve rather than straight to accommodate the untwist per-span with speed.