If you're not running your own mail server, then you probably don't need one. Reverse DNS gets used for various 'techy' things. The main one that springs to mind is to verify that your IP address is who it claims to be, if you have a mail server running on the IP and are using it to send email. Some mail servers check this when receiving email from another server, so if the sending server doesn't have a valid reverse DNS record, the receiving server may be configured to reject email, which will obviously cause issues.
There's probably a few other uses for it that I'm not aware of, but in general, the average end user doesn't need to have it configured, or indeed worry about it that much.
Hope this answers your query.
Edited by mixt (Sat 19-Mar-11 17:40:54)