Putin desperately had to be seen, in Russia, as doing something that seemed to indicate strength, in order to be able to retain power. Facing off against NATO, and getting them to offer him concessions, served that purpose. Invading another country served that purpose. Holding bits of Ukraine, and the fates of people in them, are very much incidental. The concessions were not necessary, but the offers of concessions were all-important.
Putin desperately had to be seen, in Russia, as doing something that seemed to indicate strength, in order to be able to retain power. Facing off against NATO, and getting them to offer him concessions, served that purpose. Invading another country served that purpose. Holding bits of Ukraine, and the fates of people in them, are very much incidental. The concessions were not necessary, but the offers of concessions were all-important.