Wood is heavier as has been said, but it does bend into the job. The older guys will know what I mean. Say you have a window over a porch. You can put a wooden one at a bit of an angle and get nearer to the glass. When you go up, there is enough flexibility in the frame to allow the ladder to twist slightly. both feet will still be on the ground and the top of the ladder flush with the wall.
I have both auminium and wooden ladders with alloy rungs. I prefered the wooden one for everyday use though. Dai