Leverage, I think. He can extract a lot of concessions in exchange for Kiev. I suspect Russia would trade it happily for the Ukrainian coast, which is strategically more valuable and has a plurality of ethnic Russians.
Also they probably want to destroy/capture military central command.
I think he thinks he has nothing to lose at this point. The EU is looking for alternate suppliers for gas, and when that's in place the sanctions noose will be fully tightened.
The only thing he can do to the EU is triggering a huge refugee crisis by terrorising the local population. Excluding Crimea/Luhansk/Donetsk 38 million people live in Ukraine. The cost to the EU would be huge.
Also they probably want to destroy/capture military central command.