Probably both. Divert pressure off the front. Disrupt supply lines and infrastructure (roads, bridges and railroads). I’ve also heard mention that one axis of advance is towards a nuclear plant.
I’ve seen some keyboard warriors theorize that the Ukrainians intend to capture the plant as a bargaining chip in negotiations with Russian because Russia currently holds a Ukrainian nuclear power plant. My (wild ass) guess is that if they make it to the plant, they will just destroy its ability to generate power, then retreat.