Can someone explain to me why they need port forwarding functionality through a VPN?
The problem is inbound connections. If both peers are behind NAT they can't connect.
And I run services through it that I want access to from outside my subnet.
Which is not a lot because in most countries exposing your IP on the torrent leads to legal threats.
For this reason, your selection of peers will be limited. As all other peers behind NAT without port forward will be disregarded.
If you do have a port forward, other peers can connect to you, thus having all peers available.