Rails, like said, are to mount extras on, like a follow focus or mattebox or a big ass lens. Mattebox is a bigger lens hood but you can insert filters into the mattebox like ND or Polarizing filters. Matteboxes IMO aren't needed unless you're shooting an big scale short film or something, they really won't help out an amateur and they're fucking expensive.
That being said, rails on the other had could be nice for you too add a follow focus or to build a shoulder mount off of, either way you don't need either of the things you mentioned, they just help out.