1. "Common law rights exist only for the specific area where the mark is used" (vs registering gives you "a legal presumption of your ownership of the mark and your exclusive right to use the mark nationwide").
2. Registration grants you "the ability to bring an action concerning the mark in federal court". (It's not obvious to me how this differs from common law protections, but that's what the document says.)
Also mentioned in the document is that if you fail to defend you rights and the trademark enters common usage (e.g. "escalator" being the canonical example), then you lose protection under the law.
At any rate, the point is, this is trademark law, not copyright, so don't assume what you know of copyright law applies (or that your licenses which are written to protect copyrights will adequately address trademark issues).
