A VPN is useful for organisations which have people working from home. It provides a secure channel from the machine in the field to that organisation's network.
Enter a set of credentials (there's usually a hardware token involved) and the systems work just like you were in the office. If you want to access the internet, it is the company's internet link you would use.
The companies selling VPN services to the public have two selling points.
The first thing they plug is privacy. This, for the most part, is scaremongering. Provided that the "https://" prefix is used, and the little padlock icon is visible, the traffic is fully encrypted, just as it would be if you paid for their VPN service. In theory, a man-in-the-middle attack could determine which websites you were accessing, but figuring out usernames and passwords is NOT a simple task, despite the VPN companies' claims.
The second feature is the ability to pretend to be in a different place, just as a company's VPN lets you pretend to be in the office. Thus is useful for sites which demand that you are in a particular location before you can use their service. This is common on media streaming services such as NBC and BBC's iPlayer. If the VPN company has an "other end" in the right country, you can use the VPN to access that service. I understand that Netflix restricts content based on where you are.
Banks commonly reject card use if the transaction is in an unexpected place; I commonly ring them before holidaying abroad. If you've just used the ATM in the high street, a valid login from California would be unlikely.
So unless you intend to use media services in foreign countries, I would save your money.
