I’ve heard about a “poor man’s copyright.” What is it?
The practice of sending a copy of your own work to yourself is sometimes called a “poor man’s copyright.” There is no provision in the copyright law regarding any such type of protection, and it is not a substitute for registration.
That's right from the copyright office.
And more right from the same FAQ at the US copyright office
"Do I have to register with your office to be protected?
No. In general, registration is voluntary. Copyright exists from the moment the work is created. You will have to register, however, if you wish to bring a lawsuit for infringement of a U.S. work. See Circular 1, Copyright Basics, section “Copyright Registration.”
From stopfakes.gov
Do I Have to Register My Copyright Claim?
Last Published: 7/16/2016
No. In general, registration is voluntary. Copyright protection exists from the moment the work is created (fixed in a tangible medium). However, registration provides important benefits, such as proof of ownership.
In addition, copyright owners who have registered their copyright have additional remedies if their copyright is infringed. Visit the U.S. Copyright Office for details about U.S. copyright law.
The key is proof of ownership. If you post a song to to a website with X date then a thief would have to prove that they have an earlier date that can be affixed to the song. Now would it be easier for you to prove them wrong if the song were registered? Perhaps. But the copyright allows you no legal assistance...nothing but readily accessible documentation of the date you registered it. Would that be stronger in a court of law? Beats me. All I've been trying to point out is that the copyright office does not copyright your material. And in this day and age their documentation of registration may be an antiquated mechanism given the multitude of opportunities to affix dates to it via the web.