Oh, that's easy, for Russia it's crucial for reasons... it was something about de-nazification, or was it to stop NATO expanding? No, no, it's to defend against the aggression of The West!
That's why Russia has to occupy Ukraine!
For Ukraine, they mostly seem to have a bee up their butt about Russian troops occupying, torturing, kidnapping, displacing and murdering Ukrainians on Ukrainian soil.
Soviet Union was big, included many states. Socialist economy imploded, Soviet Union became Soviet non-union. States declared sovereignty as independent countries (mass brexits, kinda?).
Russia, who used to be able to socialize assets from all the smaller countries, wants to be able to socialize the assets again, and as a bonus, is salty because some of those countries are hooking up with NATO, which is by and large a union of capitalism...
Instead of trying to create a union again, and getting others to join via diplomacy or negotiation, Russia is just trying to press its neighbors into joining the reformed union via military occupation, and hit a slight snag with Ukraine, because all this shit is a little too recent for Ukraine to have forgotten how shitty a lada is.
And that in itself was another reason to invade. A free and prosperous western-facing Ukraine might cause the Russian masses to begin wondering why they can't have that too. Putin cannot abide that.
Thank you for making a serious and accurate reply to a pretty silly post. For real. Others may see it that actually do not understand what it's all about.