There are several reasons of the importance of Crimea to Russia, the primary of which is the Sevastopol naval base, which is the largest Russian naval base on the Black sea:
- It is a comfortable harbor with extensive military and naval infrustructure, built over decades (or even centuries). It is simply difficult to replace at a reasonable cost and in a short amount of time.
- It allows Russia to control the black sea coast of its immediate neighbors, notably Ukraine and Georgia, but also other countries, and potentially teh Balkans. (See below for the map).
- The Black sea provides the shortest and the surest access to the Mediterranean sea, and by extension to the Indian Ocean and Southern Atlantic (what in US parlance is called projecting military power to the Mediterranean).
The OP suggest that Sevastopol is not indispensable for trade - this is correct, given that Russia still has a rather long Black sea coast line. Also, traditionally the trade in the Black sea was passing not through Sevastopol, but through Odessa (a Russian equivalent of Marseille), which is not in Crimea and still under Ukrainian control. However, any trade is largely dependent on presence of a military force, guaranteeing its protection, although this is arguably not the principal mission of the Russian Black Sea fleet.
Historical and cultural significance
Crimea was conquered by Russia from the Ottoman empire by Prince Potemkin (during the reign of Catherine II), in realizing the Peter I vision of Russian access to the warm Black sea.
Several wars were fought in Crimea against Turkey and its allies, most notably the Crimean war, in which Britain and France militarily supported Turkey. This war is sometimes cited as the first mediatized war, as the war correspondents were present in place, and the photos were published in newspapers, and people were shocked by the explicit brutality of the war. Sevastopol is still a name of many landmarks, e.g., in France, in the memory of this war. Sevastopol sketches is a famous series of stories by Leo Tolstoy (the author of the War and Peace) about the war.
In less formal culture Sevastopol is well-known as a hot sea resort during the Soviet period (a kind of Russian Côte d'Azur), as well as for the wine making tradition - see Crimean wines.
Some additional reading
Crimea's strategic value to Russia
What is Russia doing in the Black sea?
What makes the Black sea so strategically important?