My view is that this is about maintaining a fading colonial empire. Ukraine has been drifting away from Russian influence at least since 2014 and since Russia does not have the economy to re-assert its influence through trade it has to do so by military means.
I have no facts to base it on other than news propaganda, but I think they are too late to re-capture Ukraine and this rather desperate move will only bolster the Ukrainian nationalism. The Russian forces also will not be able to hold the Ukraine well enough to extract any sort of economic benefit from it. 45 million is a lot of people.