What does this monitoring achieve, though, exactly? If the parent is correct in that a high end eruption at Yellowstone would kill >50% of the US's population, saying "hey guys, it's gonna erupt soon" doesn't do all that much for us.

Now, if our monitoring of earthquakes was advanced enough that we could say, "hey, San Andreas is gonna go nuts approximately 3 days from now in this general area", we could meaningfully evacuate people and save lives. But we don't have that kind of prediction accuracy.