There are many reasons.
Lights dimmed
1. The lights are dimmed to improve the PAX night vision.
2. The lights are dimmed so you and the cabin crew can see out.
3. The lights are dimmed to reduce the demand on the generators and thus improve engine performance if a go around is needed.
Blinds open
1. To allow you and the cabin crew to see the lights outside
2. To allow th eCC to see any problems in the event of an evacuation.
3. To improve th echance of the tower/other aircraft seeing your a/c on the ground (300 lights are better than 3!)
4. To allow the emergency crews to see in before they start cutting your head open in the belief that no-one was leaning on the window.