Pax and cabin crew need to see what's going on outside in case of an emergency. In case for example it is not possible to evacuate from one side of the plane (because of a ravine or something like that) it's easier to see when the lights in the cabin are dimmed.

Its so your eyes are adjusted to light conditions outside.

Imagine going from a brightly lit cabin to the ground where its dark in a few seconds down an escape slide?

You would be blind, like when you first turn out your bedroom lights at night!