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.
The official SCP Foundation content is more about establishing the basic concepts and descriptions of the SCPs. Fanfiction, on the other hand, takes those ideas and runs wild with them. Fanfictions can explore different scenarios, character relationships, and backstories that are not in the official content. For example, in official content, we might just know the basic properties of an SCP, but in fanfiction, we could see how it affects a specific character's life.