My kid is in RMSC and they use the kickboard a lot in practice. This is 8U.

Yes, its about evaluating the kick. You need a strong kick, not just to pull with your arms to swim well.

How strong their kick is independent of using arms- it’s actually very important

Maybe they are looking at their endurance as to how well they would be able to swim at practice?

Just curious, my DC was recently at a tryout for a swim team and the kids were asked to swim all 4 strokes plus IM. Then they had them use kickboards with freestyle kick for quite a while. Is there something they can ascertain from the kickboard drill that they could not from watching the regular strokes? I am not much of a swimmer so I really have no idea about this.