You know how in some fairy tales, fairies come and save the day? And some fairies are bad too, like when a steel sword kills the bad fairy. Also the good fairy scares away the bad bugs in the fairy tale, which explains it's resistance to bug. The fairy protects you from the dark and bad guys, which explains it's dark and fighting resistance. For the fire thing, the fire burns the fairy, but the fairy can't hurt the fire. Hope I helped!
The fairy type is used to balance everything out