I agree it isn't much of a motivator since Astronomy is generally already given in content that gives Causality. I feel the suggestion is more about not wanting to "waste" the rewards that are capped, for people who are still doing the content maybe for fun?

I actually don't do any content that awards only capped tomestones while not also rewarding uncapped tomestones. But then again I am very casual and there are big parts of the game I don't even touch.