Currently we’re only allowed to use underscores, letters and numbers in our filenames (correct me if this has been changed for versions over 4.8–I haven’t seen any change logs about it). It would be great if we could use more than that for organization purposes, such as spaces and other symbols (mainly !, [], (), .).
I would assume its not there for multiplatform support, and those symbols may have special meanings on one of the many platforms UE4 supports.