Looking at the map, Russia taking Siversk will close off a Ukrainian defensive salient and consolidating Russian lines in the East. In the West, Russia will bypass to the North of Odessa and connect up with troops in eastern Moldova, cutting off the southwest chunk of Ukraine including the port at Odessa.
Russia is incapable of operational maneuvers like that.
If they could do it in Luhansk, they would have, it is vastly less costly than what they have been doing.
They can at best nibble away at a rate of a kilometer a week.