Ukraine war: Why is Mariupol so important to both sides?

From a military standpoint, Mariupol is the last area the Russians needs to secure a land corridor all the way from Russia proper to Crimea," Ed Arnold, research fellow on European security at the defence think tank RUSI, told Sky News. "The fact they haven't been able to do that in 56 days is probably a major source of embarrassment for them. They weren't expecting the amount of resistance they've had from the Ukrainians."