My guess is that it will be used as a pretense to escalate the war (maybe even to go nuclear)
So why annex? So Putin can claim (erroneously) that Ukraine is now attacking Russia, rather than the other way around. He could argue (erroneously) that the use of tactical nukes or some other sort of chemical or biological weapon is justified as a means to protect his "border".
Make no mistake. Vladimir Putin is "all in" on this war. There is only victory for him. There cannot be defeat. Many believe that Putin will go to any means (including chemical, biological, or nuclear) to win. It's hard to imagine him losing this war and maintaining his power in Russian. It's win or else.
Let's face it. He is not the sort of man to go down quietly.
