He attacks a nuclear plant and creates the risk of a meltdown. The result will be to remind the EU about how scary nuclear power is and ensures public opinion stays firmly against the one power source that can wean them off Russian oil.
Also, it has strategic value as limiting power to the rest of Ukraine makes it harder to wage war. No power, less internet and communication, slowing the stream of awful (for Putin) pictures and videos of his brutal war.
And capturing it would be a propaganda coup at home. Putin stopped the evil Ukrainians from blowing up a massive dirty bomb.
Pure evil genius.