If you level a city you just removed 95% of the reason for trying to capture that city.
Spending untold billions of dollars and tens of thousands of lives for a pile of ash would never pay itself off.
The reason for Putin at this point is more reputational than inherent value of whatever lies in Ukrainian city. I fully expect Putin would nuke the ever living shit out of Ukraine into a glass worthless pile of rubble if he thought he could get away with it.
But this is exactly how russians are capturing Donbass: each sity they managed to capture for the last two years are total ruins.