Well, for one, if you have any intranet services that you want to access from outside of the network, but aren't sure in their bulletproof security, it's better to firewall everything other than one port on one device for the VPN, and connect via the VPN to access intranet services.

Second, some ISPs offer TV service on mobile devices and even set top boxes, but only inside your LAN on your assigned IP address. My ISP offers up to 3 TV STB devices (that run Android TV) per contract for free (mandated by law, because I can't buy my own STB and get a smart card!), but they only work on my LAN.

Since I live away from my parents, I wanted to have TV in their house without paying twice (that same ISP is not available at my parents' house at all, anyway)

My solution was to install OpenVPN Connect on the set top box, set it to auto start on boot, and to auto connect to my VPN.

From the TV app's point of view, I'm in my LAN, and it can talk to my modem on it's fake "virtual" IP address, and also reach the ISP's servers with proper authorization (they authorize users based on the IP address that was assigned to that user, which is stupid if you share your WiFi without having VPN on the guest SSID, but whatever).