There are a shit ton of Russian citizens living in Crimea since 2014, many were intentionally moved there to firm up Russia's claim of ownership on the peninsula. If Ukraine surrounds the city from its west and destroys the bridge over the Kerch Strait, all of those Russian citizens are now trapped. They become bargaining chips. Ukraine offers safe passage for them back to Russia if Russian forces leave Luhansk/Donestk. It would also be major egg on the face of Putin if Crimea gets cut off even if negotiations don't begin right away. The Russian Black Sea Fleet is stationed there at Sevastopol. Sorry I wasn't very clear about that part.