Trivia: You can hear the Wilhelm scream twice during the scenes in the restaurant. The first comes just after the Bride fights Johnny Mo after the screen turns black and white. The second comes when the Bride races up the banister (during the "Nobody But Me" song). She slices a guy, then shoves him and he rolls backwards down the stairs.
Trivia: When the bride has the sword through one of the '88's", and is somewhat "dancing" with him, turning in circles, whilst the others are moving out of her way - the music being played is the same music from the movie "Gator" with Burt Reynolds, during the climatic scene at the end at the motel.
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.