A virtual private network (VPN) is a way to connect to your office network from a remote location and do so securely. It’s “virtual” because once connected, it’s as if you are virtually in the office, able to access the same servers, printers, and other network resources. It’s “private” because all of the traffic between your computer and your office is encrypted, so it’s secure from eavesdroppers even though you’re traversing the public Internet. You can think of a VPN connection as a secure tunnel through the Internet to your office.
The short answer is security. A VPN provides a very secure means to connect to your office network without exposing your entire network to the Internet. Some common uses for VPNs:
Of course, there are other means of accessing your network resources besides VPN tunnels. For example, to get to your accounting application remotely, you could provide a direct connection to your accounting server via the Internet. But even if it’s password-protected, you’ve now exposed your server to the entire Internet. And it’s piecemeal: if you want to provide remote access to any other applications, you have to expose them as well. Pretty soon your Internet firewall has so many holes, it’s like a house with all the windows and doors left open.
If, on the other hand, if you provide VPN access to your office, you can provide general access to your entire network via the same connection, and you can enforce better security through domain authentication, security certificates, and other means. It’s like having a single door to your office that everyone has to go through, and it has multiple locks on it.