The architectures of the internet protocol TCP IP can be difficult to grasp. The complexity of the protocol extends beyond the way packets are sent from computer to computer, even making the way a computer connects to a network a mystery to some. In order for a computer to connect to a network and subsequently other computers, it needs to carry a unique address, an IP address, that allows it to be identified on the network. This lets the network properly route data to and from the device, and since only a single computer may have an IP address, the data will get to the correct recipient. Computers need to contact each other for a variety of reasons.
The most popular reason for a computer to connect to another is to download a web page. When a computer user connects to an internet site, their system makes contact with another computer, a web server, which then sends the requested data to the IP of the client initiating the connection. In some cases, a computer will connect directly with a peer to share files. For computers on a private network, this creates the need for vpn traversal to cross over a firewall to a NAT address that sits on the inside. Why do you need vpn traversal to connect to a NAT address? That is because an address behind a firewall is only valid on a private network. Those computers cannot use that address to connect outside, and they share a single IP that is assigned to a master internet connection.
When using one of those internal addresses, vpn traversal is a fix for connection types where two systems need to make a peer to peer connection. Vpn traversal is a necessity because this type of connection requires a valid outside address in order for the communication to take place. The internal address would normally invalidate this, but vpn traversal allows the connection to be made despite the NAT address.