"Important"lol. There is big difference between an uninhabited flat buffer zone where Russia is conducting a flexible defense and Bakhmut, an extremely heavily fortified and strategically important urban area the Ukrainian military defended to the death.
Should I mention that yhe Russians don't have a flexible defense.
They have a defense in depth that is layered to allow maximum concentration after what would be basically a no man's land, should they have the artillery to make use of it.
That is an important aspect as we truly do not know how many defensive limes there are, and usually three is the normal. The third is often the strongest if not the second ti allow the third as a fall back point.
Learn tactics and strategy