Strategic Naval Base. Ukraine’s only real economic port. Natural resources in the area. Quite a bit of industrial infrastructure.

Without it. Ukraine is crippled in terms of its economic activity and ability to defend itself. It no longer has a place to base its fleet. Not to mention the majority of the Ukrainian Navy was based out of there and was captured at anchor. Only a small portion of the fleet was either deployed or escaped capture. Some of the outdated vessels were returned later.

Crimea itself was formerly part of Russia. After being annexed in 1783. Soviet reorganization of internal borders in I believe it was the 50’s saw the land given to Ukraine. I guess Soviet planners at the time didn’t foresee a break up. The population is by and large ethnically Russian. Following several centuries of standard Russian Imperial and Soviet Policy of displacing portions of the native population with Russians. The largest minority in Crimea is ethnically Ukrainian.

Basically the entire situation regarding the peninsula is a political minefield.