Hello.
For which purpose this port?
(http://oi63.tinypic.com/35nc6kx.jpg)
It's an expansion port. Games could read or write to the pins in it so that you could make hardware that are used with games and connected through the port. I also think it has audio output but I'm not sure. No such hardware was released though. It's quite common, remember the expansion port on the NES (not Famicom) and one of the expansion ports on the Gamecube were never officially used by anything either.
Thank you
You can get the the combined internal and expansion audio from the FDS Expansion Port. The other pins can be used for communication from and to RAM Adapter's controller chip as a 7-bit I/O port.
Nintendo has a long history of unused expansion ports, at least in the US. NES Expansion Port, SNES Expansion Port, N64 Expansion Port, GameCube Serial Port 2, no love to any of them.