I mentioned this in another topic
but some midi devices need the ability to select the correct ‘port’, as they have multiple.
BUT since you mentioned output.
I thought Id point out that SOME midi devices will need to use a different port for input to output.
(I know, I know, usb is bi-direcitonal… but for some reason, some devices have an IN and OUT port 
)
anyway port selection is actually very useful for lot of other purposes, as its a way you can connect multiple midi devices using midi-hubs (rather than usb hubs)
(e.g. iConnectiviy mioXM)