In physical world, it is a geographical critical and strategic passage. Armed force is able to control what is allowed and what is not for passing thru.
In cyber world, similar concept is deployed in network perimeter controlling data traffic what is allowed and what is not in reaching the destination node(s). Source ports don’t matter.
The camera aperture is the good metaphor. Light sources don’t matter. What matter is to control the incoming lights from whatever directions to reach the camera senor for composing an ideal photo.
I came across a cybersecurity practitioner who is so innovative to request controls of the network source ports in the firewall as well. This involves application logic and configuration changes yet the effectiveness to enhance cybersecurity is really in doubt.