The urban myth is that the poor man's patent is to send yourself a sealed envelope with a description of your invention. If Mega Corporation then comes up with the same idea, or rips off your idea, you can prove that you invented it first. Because look, the envelope is sealed, and that proves its never been opened, right? And it has a postmark on it to prove when it was mailed, so that's a slam dunk. Right?
Wrong! What this myth is centered around is that in the U.S., unlike other countries, the first person who conceives of an invention gets the patent on it. However, that conception has to be followed by due diligence to reduce it to practice. Filing a patent application is considered reducing it to practice. So you can't conceive of an idea, then do nothing for 5 years, and still get the patent over a person who filed during the 5 years you were asleep. However, there are times when you acted with due diligence, and someone still filed a patent application one day before you did. Then you want to be able to prove when you conceived the invention. Then whoever has the best evidence of conception gets the patent, after a long and costly contest to prove first conception. It's far easier and cheaper to the be first to file, than to have to prove first conception.