There's a lot of bathroom action in Pulp Fiction. There's even more speculation about why there's a lot of bathroom action.

It's one of Tarantino's favorite settings in his films. Vincent seems to spend half his life there. Characters in Pulp Fiction go there to read, snort cocaine, hide from killers, and talk to themselves in the mirror.

In fact, characters usually come out of the bathroom to find that their world has imploded. Vincent comes out at one point to find that the diner's being robbed and Jules has a gun pointed at him. Another time, he finds Mia dying of an overdose. Brett's friend comes out to find his buddies dead and Jules and Vincent miraculously alive.

Sharon Willis, Professor of Visual Studies at the University of Rochester, sees the meaning of the bathroom as as a place where we get dirty and we clean up—two important themes of the film, doing bad stuff and then getting redemption or grace. She also views it as a metaphor for "getting caught with your pants down," like being totally unaware of what's about to happen to you (source).

And that's exactly what Tarantino wants his audiences to feel like when they watch his movies. One minute we're watching some conversation about whether miracles happen or not, the next we're seeing Marvin's brains splattered all over the car.