1. Time. The more time, the better for Ukraine. More supplies for the actual pitched battles, and more time to start stockpiling for their insurgency. At some point when Kyiv is either going to be totally destroyed or "surrendered", you of course "surrender" it, and then start killing the occupiers. Killing occupiers pretty much works every single time. It might take years, but eventually the occupiers leave.
2. "Good Faith". For Ukraine, the entire argument is that they weren't attacking Russia, they weren't trying to join NATO, they weren't being the bad guy. So, negotiations show good faith. And Ukraine's primary goal is to show the entire world their good faith so they can get more time.