RDNS pretty much is a method to determine a hostname with it's assoicated IP. Some 'free email' providers like AOL, Hotmail sometimes rejects mail from a server if it doesn't have a RDNS Entry.
In other uses, reverse DNS can be used to determine the real upstream of a reseller as well, thus many providers block reverse dns lookups for their resellers.
A reverse DNS request is the same on a VPS as it is ona regular server. A VPS is a Virtual Private Server. Being it acts just like a normal server there is no difference if a reverse lookup is done. When you do a reverse lookup you resolve and ip to a hostname the opposite when you do a forward lookup. Anytime you go to a website you do a forward lookup reverse is the opposite. Tons of email servers block servers that do not have a reverse entry like other people previously said. This is basically because of spammers.
If you want reverse for you IPs, you can setup a DNS server. You may need to ask your provider to point the IP block to your DNS server so you can handle reverse. Some providers may handle the reverse for you on their DNS servers.
Well some can see it as a way to retrace your domain to your IP or versa.
Generally it helps find a reverse (path back) to your server. However in broader sense email service providers scan RDNS and make sure emails received from "you" are actually from you and not from a forger my confirming that the domain of the email (yoruname@domain.com) is actually from the same IP where your domain is located.
Its like sending a postcard to a friend. He checks the address on the postcard and confirms it matches your address - so he knows its from you and not a forgery.