If Ukraine ends up significantly decimating the Russian fleet currently in the Black Sea, it exposes the occupied territories for attacks.

Not that Ukraine is going to take back any land this way but the attack surface is way greater than before and Russia has to either move essential assets far from the coast or they have to move resources from the existing front line to new areas. There is nothing for Russia to gain by diverting resources to the coastal lines since they will either just be able to hold their positions or lose ground. They can never advance.

Of course it all comes down to what resources Ukraine gets their hands on in the coming months and while all of the above looks promising, the reality is significantly darker.