I am not doing that myself and I am not using VQ, but I understand that people may have webserver, VPN server or things like that, hosted on the router or another dedicated server at home network. You know VQ has many power users and probably are the most tech savvy in average, among all local ISP users.
A very simple use case is to have remote access to the router, for example, my Asus RT-AX82U has this feature.
https://www.asus.com/sg/support/FAQ/1000926/
VPN server can be useful as well for remote access to home network. Different VPN servers may use different port.
Thanks for helping me understand!