1) Short term – To punish Ukraine for overthrowing a corrupt Russian puppet, and recast the anti-corruption narrative as an anti-Russian one. (This prevents Russians from following Ukraine’s example.)
2) Medium term – To prevent more successful states (like Estonia and Finland) from emerging on Russia’s borders and demonstrating property rights, rule of law and the associated prosperity to Russians.
3) Long term – Russian history is a lie. Until Czar Peter, in the 18th century, decided to call the Muskovites and all the conquered people “RUSSIAN”, there was no such thing. They stole Ukraine’s history. The ancient RUS people are Ukrainian. Russia has an identity crisis without Ukraine.