If you're doing a P2P related activity over a VPN (or otherwise), port forwarding is very important for improving speed or enabling the service at all. That's because your router blocks incoming traffic from certain ports by default, ports that will be used with a P2P connection. To get around this, you can 'forward' a port that can be used for said P2P activity, letting your router know that the traffic you expect to see from a specific port should be let through.

You can simply leave port forwarding to your personal router, but if you want to stay anonymous while participating in P2P connections, then you'll want to use a VPN service. If a VPN service doesn't utilize port forwarding, then any P2P connections you use will either be straight up impossible, or very slow. For example, you wanted to host a gaming server without giving away your actual IP address, then a VPN with port forwarding is desirable. The same can be said for torrenting.