It forwarding opens a port direct to your system from the vpn ip.

So if your internet address at your router to your ISP is 1.1.1.1(which NATs it to your PC address on your lan) and the vpn exit address is say 10.10.10.10

Then when you port forward a port, say 443 (you wouldnt) then any request to 10.10.10:443 would then go to 1.1.1.1:443 and then to your pc on your lan.

In short it allows a direct connection through the vpn

Edit

Technically that's slightly incorrect,

packet goes on 10.10.10.10:443 but it goes to the PC directly through the vpn tunnel. The port it goes through the 1.1.1.1 router to the PC on could be anything, it won't necessarily be 443. It will be whatever the vpn is set to use