alt Hacker NewsIt’s a feature that doesn’t take much die space to implement, so if they didn’t need those pins otherwise I don’t see why not to add it. If it turned out that the PPU wasn’t good enough for what developers expected, this would have let them make a quick follow-up console as a stop-gap with little R&D expense required. If you want an actual example of overengineering on the NES, they put a giant custom connector on the bottom of every system that never ended up getting used for anything. They probably wasted at least a couple bucks per unit on that.
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As 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.
They did use the connector on the Famicom multiple times, so it's not a complete waste. They just didn't end up bringing the Disk System or Satellaview stateside.