What, compared to plate tectonics? What utility exactly does plate tectonics have?

It doesn't have much utility, but it changes the way we understand the world in a pretty big way. Not revolutionizing physics big, but turning the immense, eternal and unchanging face of the Earth into something that moves and flows in deep time is pretty impressive viscerally. Adult neurogenesis just doesn't seem as big, even though it probably has more utility.

Viscerally big things are ones which draw the attention of the people skeptics debunk, so that's why I'm picking them out here.

For that matter, how could sitting down and doing nothing whatsoever or reciting some mantra like a Buddhist monk improve your health and happiness?

If a brain does focused relaxation, it gets better at relaxing. I don't see how this would have been very counterintuitive 50 years ago. The unexpected part was the causation going from mental actions to brain anatomy.

Now I'm actually interested about the history of this concept. The correlation between mental abilities and brain anatomy has been entertained for something like 200 years. Abilities getting very much better with training has been known forever. So when was the idea that training a skill could actually change the relevant brain anatomy to a degree first introduced? I've no idea.