Ukraine

I suspect there's multiple reasons. There's morale, there's increased control of the Black Sea, and there's cutting off supplies and other support for the southern front. I think the last is the most valid reason.

Ukraine more or less has drone control over the roads running east-west along the southern front. This left the Krim as the safest supply route. However, the Krim is relatively easy to isolate because it's an island. Destroy the bridges and ships, and voila, no more supply line.

Additionally, Russia used the Krim as a safe rearguard and to launch long-range strikes. Displacing such strikes gives Ukraine more response time, because Russia needs to launch from further away. And it makes it harder for Russia to strike Odesa, which is important for Ukraine's exports and therefore also for its financial situation.

If Ukraine manages to break through Russia's southern front, I wouldn't be surprised if they can push all the way to the Black Sea/Krim/Sea of Azov in one go, since Russia will have great difficulties responding in that area.