When they build a Vehicle, they may make multiple models of said vehicle with different options as well as different luxury levels like Heated seats for example, the wiring harness they use will have all the same connections no matter the model, there just won't be that optional extra that requires the plug.
It could also be a left over from the design stage where a planned option was not seen as a benefit, but they already ordered the wiring harnesses and so, there may be one or more plugs never used.
My Car was the top of the line model back in 2008, it has many orphan plugs that were probably for lesser options or also ideas that brought the car to a price that people may not have paid for.
There will always be a power and most times an earth so that what ever option would work, not much point to have a wiring plug with no power feed

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