With port forwarding, reprehensible people downloading rubbish hollywood movies using bittorrent can download from lots more peers.
For ipv4, a home internet box normally gets one internet ip address. The traffic to and from the three smartphones, two computers and two smart TVs uses one ip address on the internet side. The process of combining the traffic is called NAT.
Commercial public VPNs mostly have lots of customers using each VPN server. There may be a thousand customers behind the one ip address of a particular vpn server. It's a big NAT. (I know of only one commercial VPN service that gives out real ip addresses.)
A computer behind NAT can connect out to a port on a remote machine, it can connect out to the https port on facebook.com or whatever.
Bittorrent clients both connect out to a remote port and listen for incoming connections.
Two people who are both behind NATs without port forwarding can't connect to each other. Their bittorrent clients can't reach each other.
In the year 2001 there were lots of people on dialup or with a single PC on a cable modem. They had real ip addresses, not 192.168.0.x They could connect to each other.
In 2001 about two thirds of machines swapping mp3 files using emule, winmx, bittorrent etc had reachable listening ports.
These days, on public torrents, it's in the ball park of 1 in 6 peers are connectable. 1 in 10 in Asia where people are commonly behind CGNAT, both on home and cellphone network ISPS. Around 1 in 2 on private trackers.
A majority of the bittorrent clients can't connect to each other, it mostly keeps working because a minority of otakus upload tens of terabytes, but there are lots of torrents with one or two seeds where the unconnectable torrent clients get nothing.