VPN's have a number of uses: I will list what I think are the most common ones:

1) Keep your internet secure, even when using unsecured wifi connections
2) Keepy your internet usage anonymous (you access the internet through your vpn's ip address, not your own)
3) To anonymize your IP address specifically for bittorrent or other p2p usage
4) To access websites or web services that are specific to certain locations. For example, you can only stream netflix from a U.S. IP address, so if you want to subscribe to netflix streaming from outside the U.S. you need a vpn.
5) To keep your ISP from monitoring your internet usage

There's a good article on why everyone should have a vpn here:
http://vpntestdrive.com/why-you-need-a-vpn/