I’ve heard people dispute whether rotational weight is real. Is it?
Yes. Image you are holding a long stick from one end with something skewered on the other end. If the object skewered on one end is a watermelon, it will be harder to wave it around than if the object is a lemon. This is an exaggerated example of what is happening when your wheel spins. The spoke is the skewer and the nipples/rim are the object. Here’s an external link with a more technical explanation of rotational weight.