It may have been about political security. If Ukraine which is basically at least part Russian had become a prosperous democracy on Russia's doorstep it would make it harder for Putin to justify his autocracy. In fact that one may come to pass.
Invasion into Russia would ideally go through Belarus, which is part of the reason that Belarus is such a critical ally for Russia, and now even hosts their nuclear weapons. Since that's not possible, the second best route (and third and forth and...) is through Ukraine, likely towards Kursk or Belgorod.
There's even something of an equal but opposite here on NATO's side - the Suwalki corridor [1]. It's a narrow stretch of land between Belarus and Kaliningrad (a Russian exclave) that, if controlled, would cut off the Baltic states from NATO. So if war ever breaks out between NATO and Russia, it would be a key strategic point and unsurprisingly, it's been heavily fortified by NATO - there are even hundreds of American troops there.
Contrast that with the European countries that actually fear an invasion: they are preparing bridges for demolition, scouting suitable areas for minefields, digging anti-tank ditches, installing reinforced pillboxes and bunkers. Last week, Latvian media reported that the government is even considering tearing up railways near the Russian border to slow the invading force.
The scenarios the Russians are preparing for include, for example, mass unrest in Belarus that would lead to Russia invading the country to keep its dictator in place, like they did in Czechoslovakia in 1968 and in Hungary in 1956. In 2020, this almost happened in Belarus over fraudulent elections and mass protests that were ultimately suppressed without requiring a "brotherly military intervention" by Russia.