Ukraine has been tenaciously holding onto the point of its Donbas salient whose tip is currently Severodonetsk. People have questioned why Ukraine has felt it is so important to keep this salient going instead of retreating back to say Bakhmut. The answer is logistics, and specifically the railroad.
Railroads play an outsized role in Russian logistics. Part of the Russian’s early failure in the north was related to their inability to seize railroad hubs such as Kharkiv. The Russians did not properly prepare to transport so much of their supplies on trucks stressing this limited commodity for them beyond their ability to supply their troops. When Russia has fought within easy reach of a railhead, they have performed far better, particularly with regard to supplying their heavy artillery.
Currently, Russia has two rail lines bringing supplies in from the North to the Donbas region, and then a major line coming in from the East. Belgorod is a major military hub for them so a significant amount of supplies will be getting routed through it, and the most direct route from Belgorod to Luhansk goes straight through the rail line found in between Severodonetsk and Lysychansk (in the circle on the map above). The railroad actually follows the river in this area and so the main line does not go into either city proper. If the Russians were to capture both Severodonetsk and Lysychansk they would add a third N/S rail line, and it would be the most direct possible route from Belgorod to Luhansk.
Now, the railroad does run on the western bank here meaning its not sufficient for Russia to take just Severodonetsk. The Russians would need to cross the river and take at least part of Lysychansk in order to complete the connection. So this establishes why Ukraine is not interested in retreating all the way back to Bakhmut. But why Severodonetsk?
As we found out earlier today, it appears Severodonetsk has been prepared as a trap. There is talk of underground complexes in the vein of Mariupol. If true, this would allow the Ukrainians to appear to retreat from the oncoming Russians, go through the tunnels, and come up into positions behind the Russian lines potentially trapping Russian forces. Cities are already deadly kill zones and the Russians have not trained for them, nor show innate talent in fighting in them. If you are flanked in a city, you may quickly find yourself with no cover at all and I’m guessing that is what happened today. It is very possible this type of trap was only possible in Severodonetsk which is more circular in shape than in Lysychansk which is more long and narrow. In addition, Lysychansk may not have the old Soviet tunnel system that Severodonetsk possibly does.
Finally, Severodonetsk is on the East bank of the river. As Ukraine has not yet demonstrated prodigious river crossing ability (though I expect they’re keeping what they can do as hidden as possible as hinted at further north), keeping a fortress on the East bank can significantly help their prospects when they counter attack. Should Ukraine have the resources to go on a significant offense, they could use this position to either drive north to the critical rail hub of Kupyansk thereby cutting off the Russian salient in Izyum. Or they could drive East to Novoaider and cut a N/S railroad to Luhansk. Which direction they go would highly depend upon Russian force disposition and Ukraine’s ability to greatly extend and defend this salient. However, cutting those rail lines would be truly significant in further reducing Russia’s capability to supply its troops in these areas and why it is so important to keep Severodonetsk if they can.