Crimea was both - it was a very potent ideological move that boosted Putin internally, but it is also the main Russian naval base on the Black Sea for a good reason.

Russia also exports food, so it doesn't really need a "breadbasket" region. Ukrainian heavy industry (which is mostly in the East!) is a more lucrative prize. But there's really no economic justification for a grab for the entirety of Ukraine. Donbas alone would be a net sink for a very long time.

There's plenty of (internal) political reasons, though. Especially if you accept that Putin really believes the tripe that he is saying about Russia and Ukraine, which I'm increasingly convinced is the case.