Intuitively say, I would say it's a "kill two birds with one missile", situation: you get a buffer state with NATO _and_ some resources (water / minerals.)

By a very naive reading, this maps [1] seems to scream: "Dombass has all the mines. It's the mines, stupid !" ; but I could not find any resource on the topic (neither to tell it's the real casus belli, nor if they're nothing else to grab there.)

[1] https://img.favpng.com/24/9/15/ukraine-natural-resource-map-...