Well - you’ve answered your own question. And correctly. You are essentially changing the LF rolloff curve, and it can make things tighter and less bloated, particularly if you are getting too much reinforcement from the walls/corners.
Ports are a compromise in most cases, a way of extending the bass response beyond what a sealed cabinet of the same volume could deliver. But that added, externally-vented bass can be very interactive with the room. So tuning it to the room/position with plugs can be really helpful.