From what I understand, Crimea is important to Russia because (mostly) of military strategy. The Russian population living there obviously helps their cause, but their main objective is the Sevastopol naval base, which is large and adequate, and can house their extensive Black Sea fleet. This gives them fluid military mobility that reaches Southern Europe, but most importantly, a quick access fo the Middle East, such as an ability to stay in touch with their friends in Syria.
Take Sevastopol away from Russia, and they lose a lot of influence in the oil-rich Middle East, influence that will be left to the West to keep to itself
To understand why one base is so important to Russia, remember how we take for granted American access to around the planet, they have bases everywhere, because everyone is their friend (or afraid of being invaded)