It seems clear that Russia's 2014 annexation of Crimea was primarily to reestablish control over the Sevastopol Naval Base from which they can project power throughout the Black Sea, and, depending on Turkish whims, into the Med.
It is also clear that Russia's initial goals have changed considerably since the invasion and now they seem to be content, for the moment, in establishing a land bridge to Crimea to. alleviate the many issues they are having in administering the annexed territory.
Ukraine can thwart Russia's goals without physically retaking Crimea. If they can retake Mariupol, break the bridge and hold it, they effectively isolate Crimea without the difficult offensive.