Contrary to current Russian propaganda, there is nothing historically that links the Russian people to the Crimea. But that doesn’t mean it isn’t a damn valuable piece of territory.
The only two navigable rivers in the Eurasian steppe that drain to a warmwater coast are the Dnieper and Dniester. Their deltas are both within a short distance of Crimea. Whoever controls the peninsula, therefore, can either threaten or guard the Hordelands’ southern flank. With Crimea ruled from Moscow, Russia is somewhat defensible from that approach. Without Crimea, Russia’s most economically viable territories are under permanent threat.
Balaklava Bay is one of a half dozen excellent ports on the Crimean Peninsula’s southwestern-face. It was once home to a Turkish naval base, which allowed the Ottomans to keep the Russians off-balance for the better part of three centuries.
For more on how and why the Russians will push into Central Europe, see Chapters 10 and 11 in The Accidental Superpower.