alt Hacker NewsAs the other reply mentions, the connector was used for the disk system in Japan. Nintendo didn't like it because of piracy and reliability issues; and then finally memory mappers rendered it obsolete.
The disk system had a much more powerful audio chip. If you have a Switch, you can hear it on the "Excitebike vs" game that never made it to the US, or you can go on YouTube and look up the opening to the Japanese version of Zelda.
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The Disk System RAM Adapter plugged into the cartridge port on a Famicom. The Famicom 15 pin expansion port is just a fancy joystick port (the console is otherwise limited to a pair of hardwired joypads).
The NES expansion port on its own doesn’t seem to have enough lines to support the disk drive, notably it doesn’t have any address lines (CPU or PPU) at all, or the PPU data bus. It seems a bit weird frankly with the choices they made, I’m not sure what it’s actually for, apart from breaking the idea of extra sound chips in cartridges.
The Famicom didn't even have the connector. It had a standard off-the-shelf 15-pin connector instead, which was mainly used for external controllers. The Disk System instead connected to the Famicom via a RAM adapter that plugged into the cartridge slot. The NES expansion connector was intended to be used for a localized version of the Disk System, but as you mention this was scrapped in favor of cartridge mappers. After this, Nintendo then planned to use the expansion connector for a localized and expanded version of the Famicom Network System modem (see https://archive.org/details/0966961706/page/390/mode/2up ) but this also ended up getting scrapped.