I have always wanted to know this and finally I found out: “The purpose of dimming lights for take-off and landing is to ensure the cabin environment matches the exterior environment. This is done in preparation for a potential evacuation at dusk or at night. Going from a bright lit cabin environment to one that’s pitch black would require some time for our eyes to focus and see the evacuation slide. Since we need to have all the seconds on our side in the event of an emergency, dimming the lights is one of many steps we take to ensure the safety of our customers.”
I got the answer from http://gofar.aircanada.com/go-far-answers. And I did not put this question, I just happened to go to that site.