They need a more secure springboard to Central Europe. Belarus has a limited border with Poland (around 80 mi.) Getting Ukraine means that you will get access to 45 million-populated country, with important docks in Mykolaiv, uranium mines in Zaporizhya. Donbass, and Krivbass, and overall a big economical industrial base. Also you get Kyiv, in which slavic people were turned into Christianity by Volodymyr the Great. Which is a big win for Russian Orthodox Church and overall narrative of Russia being supreme leader of all Eastern Europe. You also get a specialized factories in Dnipro, then Dnipropetrovsk, which are specialized on making rockets, missiles for space-related activities, and are able to produce MRBM/ICBM's. Aka KB "Yuzhmash".What makes you say that?
If Russia wants to be a superpower, it needs Ukraine. It's like that America can't be a superpower, if you take away California, Texas and Florida.