I’d have to see it in the specs, but if you open the inbound port it would allow someone who got your address from the tracker/DHT to proactively connect in and feed you data. More peers = more speed each little bit helps. I don’t know if the announce to the tracker specifies ‘I need parts A, B ,& C’ where peers could connect in offering those if they had them, but it would only be possible if the inbound port is open.
Where defiantly does make a difference is on the seeding side ‘I have it all, 100%, come get some’ only works if the door is open.