- It's tough to see and your eyes could play tricks on you.
- The cameras don't work as well so you can't document as much of the space as you could if the lights were on.
- It doesn't increase paranormal phenomena and may even enable false camera footage and photographs if one isn't careful.
- You could injure yourself or the environment by tripping, falling, or bumping into something.
I posed this question to the guys of Haunted New Jersey (Episode #65 - Check it out!) and aside from just working in the dark out of personal preference, or to enhance one's senses, the only reason they could say to turn the lights off were if the lights were creating a lot of noise. (Or in the situation of a case we are currently involved with, we turn the lights out when most of the household is going to bed for the night.)
Simply put, turn out the lights during an investigation if the situation warrants it, if not, keep them on! There's no need to be in the dark.