Ok, then it does have a reason, a "because." We're still in logic with a because: to benefit everyone. Wouldn't you need to define "benefit" here? Is the benefit survival or is it happiness or material goods or evolutionary strength or cultural worth or my tribe survives to save the world...?
There can be differing ideas of what benefit means.
Whatever these benefits are defined to be, they become the greater value that the golden rule valuable as a means to achieve.
It's survival AND happiness, which again is key to survival. It goes down to the root of existence itself - it sustains the want to be alive.
I asked you earlier, but got no direct answer: What's the benefit of 1 Samuel 15:3? What's the benefit of the killing of David's child? After all, it's part of your scriptures... is it better to ignore it as human addition, as you implied earlier, and therefore call the entire prophecy of Samuel from your god to commit genocide, as falsehood?
The Golden Rule is a tool - a tool to make moral decisions, the only tool required. Insofar as that is concerned, what it is operating on has greater value than the tool itself, but without the tool, that 'greater value' is lost.