BT protocol works thru both parties. You have seeders and leachers (called peers). Both need to make a connection but how that connection is initiated and opened is important. If a peer initiates the connection and has their ports open, you're good, regardless of your own setup.
Unfortunately not every seeder does this (for various reasons). And that's when having your ports open makes a world of difference. Because if the peer also has their ports blocked, you will never get a successful handshake between the two of you.
On torrents that have hundreds of peers, you're likely fine; they'll be plenty that can initiate the transfer for you. But when you get obscure torrents with only a handful of peers, you're likely fucked. I'm over simplifying for the sake of discussion.