For the same reason Russia retreated from it. The river makes a strong naturally defensible front. Ukraine can now shore up the Western bank of the Dneiper and make it very hard for Russia to retake the land they retreated from. And Russia can do the same with the Eastern bank. And now both sides can relocate forces to the Eastern Ukraine front. If either side didn't do this then the other would have a large advantage.
Ah, Katie Hopkins. Is she somebody who minces words? "No..."
The Russians had two reasons to keep the territory.
Optics- the look of giving up a regional capital, that you'd just annexed, over to the enemy. Bad but that's only if you care what retards on Twitter and the western MSM have to say.
Strategic- if an advance was being planned and the territory was to be used as a staging ground.
Clearly the latter has gone by the wayside, if it ever was a thing. If there is a coming winter offensive it will be in the east. They can now move forces to the Donbas and have an easily defensible line to protect Crimea.
If this war drags on for years, and it well may, then one of the more important objectives is not to fucking lose and to preserve your forces. The Russians know with a 100% certainty that there's an expiry date to the west's, especially the American's, support for Ukraine. Eventually it will become too expensive and too much of a political liability for many NATO countries to continue. The Europeans will break first, the coming winter will be instructive and then the burgers because they always fold rather than go all in. Russia simply can't give up, it would mean the end of Russia as an independent nation and the potential breakup of the federation, they've put all their chips on the table.
I've gone from believing that the Ukraine can survive as a political entity of some kind, probably partitioned, to now thinking they'll be completely destroyed.
