"So why is Turkey in any way strategic?"
One reason Russia is involved here at all, is to preserve its one Naval base in Syria with access to the Med, and its one airfield in Syria.
Those planes need fuel. The way fuel gets there is on tankers from the Black Sea, through the Dardanelles and Bosphorus, smack dab in the middle of northern Turkey. Russia's options are limited if they want to preserve those bases in Syria, and not have to send oil tankers around the long way from its North Fleet bases in the arctic.