They fire missiles from those ships. Also can prevent grain exports from Ukraine which has a major impact on the economic war.

Significant damage to the Black Sea fleet would make it clear to many in Russia that Putin is weak which makes a coup more likely. Continuing this war is not in Russia's best interests it's only in Putin's interests (admitting defeat also makes him look weak. A coup in Russia likely ends the war.

Certainly it's not something that would automatically end the war, but it significantly increases the probability of the war ending sooner. And there is significant strategic value to that fleet even if it's not directly engaged in combat.

the navy acts as mobile missile bases, they also allow Russia to move supplies by water, with there only being 2 land routes to most of the front, the ability to use shipping to bypass blockades, the sinking of the Russian fleet would also have allowed Ukrainians to more easily destroy the bridge between Russia and Crimea, this would make 80%+ of Russian positions in Ukraine untenable in terms of holding militarily, and since Putin can't afford to lose half his army in just a few months he would be forced to pull out and negotiate.