It's not just a propaganda move. It is meant to provide 2 things: - Legally allow Russia according to it's own laws to send conscripts to the claimed territor

It's not just a propaganda move. It is meant to provide 2 things: - Legally allow Russia according to it's own laws to send conscripts to the claimed territories. This is necessary due to the extreme man power shortage. - Legally allow Russia to officially threaten with nuclear force. Officially doing so otherwise is agains official Russian military doctrine. All nuclear saber-rattling before was unofficial. Formally annexing was a legal necessity for Russia because otherwise Russia legally no longer had any way to escalate a "special military operation" they are loosing according to their own declared objecives. Even a supreme dictator is still bound by the laws of their own dictatorship even if they can change those laws at will. Respecting the process is part of retaining support and legitimacy.