Bizarre
The Kremlin wants to show fellow Russians, and the world, that they (Russia) are winning in the conflict. Mariupol is a trophy/symbol of that victory, in that respect.
I guess that’s why the renewed advance (reportedly) on the Azovstal plant. There are 5 days left until the supposed parade, so I’d guess they want to get the plant as cleaned out as they can before the parade.
At the very least, make sure that the Ukraine forces in Azovstal cannot disrupt the parade.