In the video Kent says it's to separately influence the bottom part of the droplet, and the other bone that stands upright is for the wiggling. So basically just to have those two functions separated, have the base not move at all and delegate the wiggle to the upper part and control it freely. That's how you get a droplet to move to the rhythm of the Macarena.
Though he doesn't use it that much. The base one could also serve as an overall translation bone, if you want to move the droplet to another position.
I wanted to further satisfy my curiosity about the bottom bone’s function in the animation. I did two tests to satisfy my curiosity.
The vertical, animated bone is given a gradient of influence: strong at the top, minimal at the bottom of the hemisphere. The bottom bone has no animation, no movement, and is assigned full influence. So what happens if the bottom bone is removed?
The answer: Without the bottom bone the droplet slides along the surface of the grass blade. Its horizontal jiggle is unrestricted.
I think it’s fair to say, a good analog, is the bottom bone acts as an animation-holding edge, that stops the horizontal animation from going any further than the bottom of the hemisphere. And (possibly) from the grass blade’s animation from being transferred up into the droplet. The bottom bone’s full influence with zero animation effectively restricts/limits the animation from above (and maybe from below).