Maybe it's not worth it, but you may have a huge hit and suddenly find every commercial station in the country playing your song for a week. It's a strange world. For more info you can call either organization to request an info package. It establishes its existence and ownership. As far as the copyright office goes, it's probably overkill, but it's up to you.
Publishing is more of a personal or band choice.
They will then hypothetically act as an aggressive promoter and collector. This is a weird and shady world, and if you get offered a publishing deal, we suggest you talk to someone who knows more than we do before sell your soul or your songs.