You use services like NordVPN when accessing servers on the Internet. For example, when you don't want Netflix to know you're connecting from a country where their content is not available.

You would use a VPN on your network if you want to access your home machine when traveling. This type of VPN is typically available from many firewall/routers. For example, I use a Netgear Nighthawk and connect to my home network through it when away from home. Once you have a VPN connection to your home, you could use an FTP server to access files. An easier way, however, is just to map a drive to your fileserver. A VPN will make your remote machine appear as if you're on your home LAN.

If you don't have a VPN, you could use software like SynaMan or Owncloud to access your files.