The reason we either dim or turn off the lighting for take off/landing is so the crew will be able to assess conditions outside the aircraft in a emergency. If the lights were turned up there could be glare on the windows and also eyes would not be adjusted for darkness outside. The first thing we do before a possible evacuation is assess conditions outside by looking out to see what is going on (we can see things that the pilots can not) and we look for fire, smoke or water. Also yes, by having lighting turned down the exit signs and emerg path lighting will stand out but the main reason is to assess conditions.
If we go by the book on lighting it would be something like this :
1. boarding -upper lighting on and sidewalls on (dim)
2. demo -upper lighting dim or off with a video and on with manual demo. sidewalls off (they remain off for passenger comfort at this point as they bother the eyes of the person in the window seat). They crew may elect some lighting on a night flight to check the cabin after the demo (seatbelts/luggage, etc).
3. takeoff -dim or off at night and can be on during the day.
4. service -upper lighting on so we can all see what we are doing.
5. after service -dim or off on a day flight and on 'night' setting for a night flight (very dim).
6. prepare for landing -upper lighting on to check the cabin again.
7. landing -same as take off.
8. arrived at gate -upper light on and sidewalls back on the same way we boarded.
9. emergency demo (to brief pax for a emerg landing/evac) -all lights to bright... this is the only time all lights go to bright except for aircraft servicing.
This is all Continental procedures and is NOT required by the FAA. Hope this helps you out.