Well, Los Angeles is the setting of 'Pulp Fiction'. It gives the story a specific backdrop. The city's diverse and sometimes seedy elements contribute to the overall noir - like atmosphere of the movie. For example, the different locations in LA, like the diner and the seedy motels, play important roles in the various storylines.
The SCP Foundation is purely fictional. It's a collection of imaginative and often creepy stories and characters that are made up for entertainment purposes and have no basis in reality.
Crossover fanfics also attract new fans. If someone is a fan of another franchise and then discovers an interesting SCP Foundation crossover, they might become interested in the SCP Foundation itself. This helps to grow the community. For instance, a 'Star Wars' and SCP crossover could draw in 'Star Wars' fans who are curious about how the two would mix.