Since the other comments have pointed out the instorycharacter reasons , I ' ll say something about the meta reasons Undersiders Well established characters to work with Early control of the story , as in being able to follow the stations of canon Big antagonist is already there and we know his defenses , contingencies , etc . Coil A home for the protag , so the writers don ' t have to deal with Danny The TED Tattletale Exposition Device for the protag to learn about her powers and pretty much any other information the author wants her to know And some other stuff that doesn ' t come to mind Conclusion Being part of the Undersiders is great if the Author doesn ' t feel up to writing his own story , it gives them a lot of crutches they can use . It ' s great for an alt power , just so you can show your audience how awesome your thought out power is . Wards Preestablished characters that aren ' t assholes An antagonist that doesn ' t want to kill you in form of Piggot Coil still knows about you and he will try to kill you sometime Other ways of easy exposition like power testing , etc . Sophia flag Probably some stuff that I ' m forgetting Conclusion Less crutches for the Author , but still lots of things they can spin a storyline out of .