1. Registration provides the best proof on earth that you are the owner of the song, as of (date the registration is received)
2. If you don't register it and some thief does . . . the thief now has the best proof on earth that the song is his!
3. Registration is a requirement for filing a copyright infringement lawsuit.
4. TIMELY registration (before any infringement) qualifies you for additional monetary awards (statutory damages, actual damages, attorney costs, court costs,etc.)
Note: UNTIMELY registration (after your song has been stolen) qualifies you only for the “actual money damages” you can prove -- a very expensive process involving auditors and investigators. You no longer get the $150,000 award, and you no longer get your legal fees and court costs paid. Even if you win the case, by the time you pay your lawyer, auditors and investigators you may not get a dime!
5. If someone wants to record your song, they owe you mechanical royalties . . . if the song is registered. However . . .
"The licensee does not have to pay if there is no record of copyright registration filed at the Library of Congress."
— John Braheny, The Craft and Business of Songwriting.