A connection has to be established. That is only possible if one side has an open port.
So you can basically not connect to other people with closed ports, which reduces your available pool of people to connect to.
As long as there are enough people with open ports for you, you and the torrent ecosystem will be fine. But when nobody or very few people have open ports, torrenting simply doesn’t work.
https://lemm.ee/post/56692320/18526252