Is this a method to avoid VPN to the router while in remote locations?
No. It is a method to mitigate the risk that someone breaks in to your VPN which is open for login from anywhere because you do not know in advance from where you're gonna connect next time.
The bad guys target VPNs because everyone uses them without being able to assess their actual security. So if you need to keep your VPN open for login from anywhere, use of port knocking is a useful kind of additional reinforcement against possible vulnerabities of the VPN software.
Its drawbacks are that unless you use some kind of "rolling code", it provides zero protection against a replay attack taken by a man in the middle (like the IT guy in the hotel from which you connect or the guy who has broken into the hotel's AP or the guy next room who records the wireless communication of your PC which uses the WPA passphrase which is common for all clients of the hotel AP), and that if you knock from behind a NAT which randomly spreads the connections among several public addresses, your router will not let you in because all knocks must come from the same IP for the whole sequence to work.