They add more storylines. The Pokemon series has a lot of potential stories that the official series might not cover. Fanfictions about Ash and a Pokemon girl can explore different relationships, like friendship or romance, that can make the Pokemon world seem more diverse and alive.
Water - type Pokemon like Squirtle could be in a difficult situation. If there are any holes or puddles in the scary house, they might be scared of getting trapped. Squirtle would probably stay close to its trainer, if there is one, and keep its shell towards any potential danger. It might also use its Water Gun as a warning shot if it feels really threatened by something in the house.
Typically, female Pokemon become more protective. They'll find a safe and secluded place to lay their eggs. In some fanfictions, they might even seek out the help of their trainer to create a proper nesting area.
Once, I was at a concert. There was a couple in front of me who seemed to be having their own little dance party. They were completely out of sync with the music and each other. The guy was doing some really wild moves, and the girl was trying to be all elegant. It was a strange and funny sight. People around were trying not to laugh out loud, but it was really hard.
My friend and I were hiking in the mountains. We came across a family of deer. One of the fawns was really curious and started following us. It was so cute. We tried to shoo it away gently because we didn't want to disrupt it too much, but it just kept coming closer. Eventually, its mother called it back and we all had a good laugh about our little furry follower.