It proves that you are who you say you are.
Here's the scenario. Let's say that I'm trying to send spam pretending to be Gmail. I'm not Gmail, I'm just some lowlife with a VPS, with control of my own DNS.
I can set the
PTR of any IP I own to
mail.google.com. But doing an
A look up on
mail.google.com will not show my IP address, it will show Google's IPs. The mismatch means you're lying.
Update: Some history and clarification.
You need to know that none of this is guaranteed to work.
E-mail was invented in 1961. This is shortly after it was estimated that the entire world only needed five computers.
E-mail's popularity exploded in the early 80's with the introduction of the TCP/IP stack to UNIX. Back then every host on the Internet was either U.S. Government, a university, or a very large corporation. Back then everybody was pretty much friends. The first worm wasn't created until 1988. Nobody thought anybody would ever do anything malicious because it was an extremely (by today's standards) small community and just about everybody knew everybody.
There were a lot of protocols created that we, today, are embarrassed about. Among them are ftp, telnet, rsh, rlogin, nfs and smtp. SMTP, as was common with several protocols, was created with no security whatsoever. It was assumed that whatever you said was the truth, because why would anybody lie?
One day some enterprising schmuck decided to e-mail a bunch of people who didn't want it and spam was born. Since then we have been fighting an ever losing battle against spam.
The spam problem is so severe that any and all tactics that will reduce spam by even the slightest fraction of a percent means that the load on our mail servers will drop significantly. I ran a check on my mail server once and the messages it was processing was over 99% spam.
Administrators are desperate for anything that will help to even the slightest degree. Anything that anybody heard helped "that one time" becomes standard practice in fighting spam. Not that any one tactic is particularly effective. But these days we put as many restrictions on mail relay as possible in the hope to gain some ground in the fight against spam.
There's no spam panacea. But even a very basic check such as this might mean the difference in millions fewer messages sent.