We were stoned and talking about movies when I came up with this bizarre theory that I thought Pulp Fiction was about music in America. I'm very aware that this is most likely not true, but we had a lot of fun talking about it.
The briefcase is Rock and Roll. Marcellus represents the origin of rock music in America (blues, motown, soul, etc) Jules is Chuck Berry and Vincent is Elvis. They go to get the briefcase back from the nerdy white guys trying to steal it. Maybe they represent greedy record producers that profited off of Black music and screwed over many Black musicians.
The reason the guy that unloads a gun at them is unable to hit them is because legends of music can never die and will be around forever. Jules realizes this and realizes that he is fighting a pointless fight. Pumpkin and Honey Bunny happen to be British and they also try to take the briefcase. They represent the British Invasion. The British Invasion was completely influenced by the blues and older pop music. Jules lets them leave with the money, but keeps his "Bad Mother ******" wallet. (They can steal and be influenced by the older music, but they can't take a legends style. They have to come up with their own style.) Jules even mentions that he is trying real hard to be the shepherd.
Butch is introduced as a boxer being told by Marcellus to throw a fight. Butch represents new and interesting rock music that continues to come out and Marcellus doesn't like this. Butch doesn't throw the fight and runs away. They both get captured by rednecks while a country song is playing. The rapists represent country music. Country music also stole a lot from the blues and other Black music. This is shown by them raping Marcellus. Butch has a chance to escape but decides to go back and save him out of respect. Marcellus agrees to let Butch live and they truce and go separate ways. Marcellus accepts that there will always be new music and there's nothing he can do to stop it. The watch represents that as time moves on, more new music will continue to be made and they will all be influenced by what came before them.