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.

But this is exactly how russians are capturing Donbass: each sity they managed to capture for the last two years are total ruins.

That's because Donbass is seen as useless. It's like bombing upstate New York or Michigan when invading the US.

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.