Continuity mistake: When Boss Tanaka breaks the bowl on the table, a piece of meatball rolls on the table. Later when the other Japanese guy throws a hand towel on Tanaka the meatball is gone. (00:59:00 - 00:59:45)
Continuity mistake: As Earl McGraw tells his Son Number One the massacre at Two Pines house of worship must have been "the work of a salty dog," he gingerly moves in for a closer look at the carnage. Even though the side shot maintains a close view on the upper half of the body, the posture of his movements indicates that he starts stepping forward with his right leg. Yet in the immediate next shot from behind, it is with his left leg that he makes his advance. (00:18:30)
Continuity mistake: When O-Ren's young bodyguard has the mace, she hits the Bride smack in the chest more than once with the very heavy, spiked ball. But later, there is not a trace of a welt, bruise, or cut on the Bride's chest (plenty of blood on her half-zipped jacket but her skin is just fine.)
Continuity mistake: At the beginning of the fight between Oren and the Bride in the garden, Oren puts the sword up to eye level and starts to withdraw the blade from the scabbard. The shot switches to a view from behind her and we see the back of her head. The eye-level sword, however, is nowhere to be seen. When the shot switches back to Oren's face, the sword has reappeared. (01:32:20)
Continuity mistake: In the scene where The Bride wakes up, a mosquito is shown landing on her. The first shot of the mosquito it shows it landing with all legs. Then in the close-up shot sequence, you see the mosquito is missing a front left leg. (00:24:20)
Continuity mistake: Oren is in the back of a limo, with all six motorbikes ahead of her, after the final two have moved up from the back to the front, but when we see a shot of Oren, in the limo, there are four headlights behind the limo, the way it was before the bikes moved up.
Continuity mistake: In the fight scene between Gogo and The Bride, keep your eye on the ball and chain weapon. It spins in different directions depending on the shot.
Continuity mistake: When The Bride and Vernita Green are in the kitchen, Vernita mentions something like "I've got to fix my daughter's cereal." Notice when she says this, The Bride is holding her mug with the handle facing Vernita, when the shot changes, it is instantly facing The Bride - something which cannot be achieved this quickly.
Continuity mistake: In the anime sequence after O-Ren Ishii has drawn the sword out of Boss Matsumoto, the blood is splashing out of him, and up on her face. But the amount of blood on her face keeps changing. (00:40:00)
Continuity mistake: When Hanzo hands over the sword to The Bride we can see all of her hair is hanging down on her back. When it cuts a huge lump of her fringe hangs in front of her. (00:55:55)
Continuity mistake: The Bride's face gets beaten up with visible marks during the fight with O-Ren's last female fighter. But after the fight her face is back to how it looked before the fight.
Continuity mistake: The Bride raises her sword twice at the beginning of the fight with Gogo. (01:14:55)
Continuity mistake: After O-Ren chops off Tanaka's head at the Yakuza boss meeting, the position of her hand holding the sword changes. At first she is holding it with an overhand grip, then when the blood shower from Tanaka's neck ends, she lowers the sword and is gripping it underhand. (01:01:30)
Continuity mistake: When The Bride is fighting the Crazy 88s, right after it changes from black/white screen to color again, they fight in a room where the light gets shut off. Now watch close, they wait. They fight. She hits a few members of the gang. Watch the gangster in the lower left corner of your screen. He's gone. He rolls away to the left. They start fighting again. Keep paying attention to the lower left corner. You see him roll away again a few seconds later. (01:27:25)
Continuity mistake: The Bride cuts off both of Sophie's arms, but when she throws her into the hospital, she has one arm. (This mistake is only seen in the extended Japanese version of the film - in other versions only one arm is cut off).
Continuity mistake: Tanaka's severed head lands on the table and stops rolling, meaning it should stop moving. However, it then falls off the table.
Continuity mistake: In the Crazy 88 fight, when The Bride stabs a bald Crazy 88 member, at first the sword is to the left of his tie, but when we see a closeup, it's to the right of it.
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.