The forces in Mariupol are draining scarce Russian forces and material at a time when Russia desperately needs them elsewhere. When the history of this conflict is written, I fully expect Historians to say that these incredibly brave men and women saved Odessa which is the ultimate Russian prize in the South of Ukraine.
It isn't just the physical damage being done in the fighting it's the impact on morale for both the Ukraine and Russia. Its in some ways a continuation on a much larger scale of those few men and women who told the Russian Navy to go ---- themselves. It's also the political damage. Putin is doing all he can to get a 'Win' broadcast on May 9th. If he cannot capture a few square miles in an area he has surrounded for so long, it's not much of a win.