@slimboyfat: the reason is purely economical. Ukraine currently controls the flow of gas and oil between Europe and Russia and even some parts of the Middle East like Afghanistan. Same reason they invaded Georgia under Bush and Ukraine under Obama - a weak America means nothing will be done about it.
Russia and Germany wanted to build Nordstream 2 pipeline to evade Ukraine control (and thus cost) over the fuel, because after Germany’s green energy policy failed, they became highly dependent on Russian fuel. Trump successfully stopped the development of the pipeline, which caused Putin to cease his progress into Ukraine which he started under Obama. Biden restarted the pipeline on behalf of Germany and is letting them finish the pipeline, at that point Ukraine will be completely cut off from any leverage in Europe and thus Russia can easily invade and reclaim their Soviet empire.
There is a lot at play, Ukraine and the Dnieper, their major river, are a major highway of fuel and goods between Russia the Mediterranean, China (through the Black Sea) and Europe. Russia is basically claiming and threatening the entire area that goes their side of the Dnieper as Russian, that would give Russia effectively full control over the Dnieper.
Control over Ukraine means control over EU, UK, Russia and all the goods flowing from Asia and the Middle East to those countries, it’s one of the reasons Ukraine never became a NATO/EU member, it’s simply got enough leverage without them, joining them has requirements that would lose them control.
Geography is really good for agriculture as well and one of the reasons why Stalin targeted and eradicated the Ukrainians, killing village by village during the Holodomor and then moved tons of Russians in to farm after the Lenin farm policies failed resulting in mass starvation across the rest of the Soviet Union. Hence why currently there are a lot of cultural Russians in the disputed areas of Ukraine, because Stalin ethnically cleansed the area and relocated half of Siberia to Ukraine.
So there is very good reason for Ukraine to exist, it is the gateway from Europe and Russia to the world. The EU is too cowardly to do anything about it, and now the US is too.