Do you see this submarine? The Russians have just stolen it from the Ukrainian Navy, in order to "complete" their sub fleet with it. Seriously?
The sub was launched in 1967 (!), and its design was already obsolete at the time. It belongs to a class of Soviet subs called "Project 641", and it proved to be the biggest and the most ineffective non-nuclear sub designs Soviets had ever built. One simple fact about its performances: it could navigate only three days underwater!
The Russian subs under construction nowdays (Project 885) are very ambitious and enormously advanced from "Project 641", so that the so-called "completion" of the Russian sub fleet is nothing than a bad joke.
Comparison: the other incredible poor-equipped Navy in the Black Sea (the Romanian Navy) has also got only one sub, but this one is launched in 1985 ("Project 877") and can navigate underwater for 45 days. It is, of course, in the same bad shape like its (former) Ukrainian counterpart, but, at least, it's not a relic!
I now return to the initial question: it is militarily understandable, mister Putin, what have you stolen Crimea for. But what have you needed the Ukrainian wreck for, mister Putin?!