A few guesses.
Disrupt supples into Donbas by taking over the train lines.
Capture large number of untrained and unmotivated conscripts that are currently surrendering in large numbers.
It's easier to travel in Russia than in the heavily mined occupied Ukraine. Maybe they're taking the long way round?
Force a buffer zone.
Russia has been using scorched earth tactics in occupied Ukraine, maybe they'll be less willing on their own land?
Additional chips to prevent Russia from insisting on freezing the borders along occupied lands (you take our land, we take yours) during rumoured ceasefire negotiations.
Land swap negotiations.