Revealing mistake: When Gogo is fighting the bride, misses and hits herself with the mace, as she falls backwards onto the table, you can see the stunt double's hairy legs. (01:16:10)
Revealing mistake: When the swordmaker writes "Bill" on the window, take a close look. The window has been wiped before (as happens at the end of the scene) and "re-steamed" - you can see the outlines of the wiping from earlier takes, with a clear contrast between that pane and the others. (00:52:45)
Revealing mistake: When the Bride is fighting the General of the Crazy 88's he slashes at her lower torso and scores a hit, but the slash on the Bride's jacket is already there before he strikes (it is not there in the previous scenes and when he slashes it makes the sound of him cutting the material). (01:23:10)
Revealing mistake: When The Bride is calling out O-Ren, Sofie Fatale has her arm on the building's beam. If you look at her arm, right where The Bride is about to cut, there is a visible seam in the sleeve. (01:11:10)
Revealing mistake: When the Bride takes a slash at Buck's heel, you can plainly see that there is a fake heel there. She definitely cuts through the sock, so it's not the sock pattern, and there is no blood.
Revealing mistake: During the restaurant fight scene, Thurman's fight double can be easily spotted from time to time as she is a few inches shorter and a little heavier than the star. It is especially noticable during the silhouette sequece. (01:23:30)
Answer: From what I understand the reason for the lights being off is the same reason for why the previous scene was done in black and white; to decrease the amount of 'graphic violence' in the movie in an attempt to keep an 'R' rating. I would assume that they had him shut off the lights for that scene as just another method to accomplish that task.
I believe the original question was asking why was it done within the context of the film (i.e. why did the character shut off the lights) not why was it done in reality. My best guess is that the manager switched off the lights thinking the 88 had a better chance of killing the bride if she couldn't see. True, they couldn't see either but there were so many of them one could possibly have gotten to her.