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.
Well, some of the well - known SCP top stories include 'SCP - 173'. It's about a strange statue that can move when not observed. Another is 'SCP - 096' which is a very disturbing entity. When its face is seen, it will go into a rage and hunt down the observer.