If you access your computer remotely, then services like Apple's "Back to my Mac" have their conveniences; however, this and similar services are useful for individual machines only. On the other hand, if you have more than one device on your home network that you might wish to directly access, then using a virtual private network is beneficial as it places your system on your home's network so you can communicate with devices as if you were at home.
Often the implementation of this is to use a hardware-based VPN (likely built into your home router) to configure the remote connectivity to the network, or use a server software package like Apple's OS X Server to set up a VPN service. However, starting with 10.5 Leopard, the client version of OS X also contains a VPN server that can be activated and configured to manage remote connections to your network.
The VPN service in OS X is the "vpnd" daemon, that can be configured using the OS X Terminal and access to various hidden files in the system; however, there is a third-party utility called VPN Server Configurator that will allow you to set up the VPN service on OS X 10.5 or later.
While this and many more server features can be implemented in current Mac systems by purchasing the OS X Server package from the Mac App Store for $20, currently only the Mountain Lion server is available so if you have an older version of OS X (for example, on an older Mac that you would like to use as a VPN server) then you might find it difficult to purchase and install the OS X Server software.
The VPN Server Configurator contains a number of options for setting the VPN ports, IP address ranges, what users accounts on the current system are authorized to access the VPN service, and many other details. It also has a quick auto-config button that will analyze your LAN settings and give you a starting VPN configuration to work with.
VPN Server Configuration is a $9 purchase, so for OS X 10.8 users it may be worth some conveniences over Apple's $20 OS X Server package, but will likely be very beneficial to those wishing to set up older Mac as local VPN servers.