For instance, let's say I spoof a name from where I'm coming from, but it also logs my IP address, granted that can be spoofed too, but in this example it isn't.
So, what you get is someone trying to send you mail from technoslave.net, but their IP address isn't 126.96.36.199, so the mail server rejects it.
I'm sure there are other programs that use it frequently, but none that I've really come across lately, especially since my IP address at home, I can change the DNS and point it to the IP, but I don't have control of the reverse for it, and I have yet to have any problems sending mail and the like.
It's just one of those things that when it was setup ( the idea of DNS ) that if you have a forward looking address you should also have a reverse.
The place where you can get bind might have some more info ( http://www.isc.org/index.pl?/sw/bind/