If Ukraine had done that the battle would be at Odesa now instead. Forcing Putin to struggle to steal Mariupol and especially Kharkiv is humiliating Russia and giving the defenders time to fortify the rest of the country.
And it has given time for an estimated 3.6 million to leave Ukraine.
Another 10 million are internal refugees. With 25% of the population on the move, many of the eastern cities must be thinly populated by now.
Yup. Every Russian killed fighting in Mariupol is no longer free to fight elsewhere. It also appears the Russians have withdrawn some forces from around Nikolayev to support this operation, so the Ukrainians have pushed the Russians back further there moving towards Kherson.