I don't remember how large that force was, but it wasn't enough to take a city that was going to fight, not by a long shot. Pretty much everyone with military experience seems to agree with this.

Presumably the Russians also understand this. So the only reasons that force would be there would be to distract, intimidate, and to keep defenders in place. Basically, keep the pressure on and be there in case Ukraine rolled over and surrendered.

The Russians are neither as stupid as some people think, nor as smart as others think. They've made mistakes. One of them was assuming that some of the Russian speaking places they've captured would be as welcoming as Kherson.

Anyway, Russia didn't know how Ukraine would respond to an army approaching Kiev. There was some chance Kiev would surrender on the spot. If they didn't send that army then there was no chance at all.