[ndiddy]

It’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.

[StilesCrisis]

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.