What is VPN actually used for?

It's used to allow secure remote access to a network. The second paragraph from the Wikipedia article sums it up nicely:

It encapsulates data transfers between two or more networked devices which are not on the same private network so as to keep the transferred data private from other devices on one or more intervening local or wide area networks.

It effectively makes the remote machine part of host network for access to resources and other machines on that network. So you can use the company e-mail, intranet and printers for example.

Find out more on Wikipedia

In a nutshell it connects two private networks securely through a public network. VPN

Both answers are right, it can be used to join two networks together, or just to connect single clients to a network or single server over public connection using a secure private tunnel. When the connection is made, all computers involved act as if they are on a private LAN. In the past, when I traveled for work, I used one on my home server so that I could connect from my hotel room to my server and stream music off of it as if I were still at home. This was just a simple one, using OpenVPN. It of course can be used to make networks at two locations on different sides of the country for a large corporation accessible to each other as if they were in the same building.