How does port-forwarding help in torrents?

I use Transmission to download files but it says that a certain port is required to be opened. However, while this port is closed I noticed that I can still download files with no problem. But when I opened the port via my router I can see a significant difference in the download speed. I know the basics of port-forwarding and that it forwards the traffic to a certain device on a certain port. So I was thinking that my downloads would not proceed since the port was closed but it didn't. So my question is:

1. How does port forwarding help in this situation?
2. Why do my downloads speed up when port-forwarding is enabled?