VPNs allow secure communication between private networks over the Internet. With so many of the systems we use being network based, the ability to connect remotely to a system to monitor it or diagnose issues is invaluable.
The same access we use for maintenance purposes is also useful for our clients when they want to access their home systems when they're away from home. Many of the systems we use, e.g. Control4, Lutron or Kaleidescape have web management capability built in, but the power of a VPN is that when you are connected, you are effectively sat on the network of the remote site. So whether you want to check the heating and hot water, log onto your CCTV system or double check you turned all the lights off, now you can.