There is no benefit unless you apply modern psychology regarding the thought processes of psychopathic narcissists like Putin.
Essentially it goes like this:. Putin sees himself as Russia. Russia is him and everyone belongs to him in it including the Ukraine. This is part of the "allure" of narcissistics - if you're empathic you get swept up in the feeling of being part of his success and his world.
If Russia loses, Putin suffers. If Putin suffers then why does he care if Russia suffers - Russia is him. So if he's suffering, and Russia is suffering, why not make the world suffer in response, and inflict MAXIMUM suffering on everyone else. So thermonuclear war and the end of everything is an option for him to that end. To punish everyone for his suffering. And thus the end of everything because of one pathetic man and his fragile ego.