Hi there,
They switch them off or dim them as during the 'take off or landing roll' you may be required to evacuate. Should you need to evacuate, your eyes have been ajusted to the level of light outside. The purpose is so that if its dark outside, its dark inside and light outside light inside meaning that should you need to evacuate (unlikely and god forbid!) you dont have a blinding effect. ie. stepping out from light into dark which would make your eyes funny.
ALso the reason the window blinds must be up for take off/landing is so that should you need to evacuate the cabin crew can see outside and see if theres fire/smoke/obstructions so that they know whether to block off an exit and redirect you to an exit which doesnt have dangers outside.
If someone can explain it better then off you go as not very good at this!