If you look at the map, the Kursk military region serves as supply route for the russian invasion army. If Ukraine army successfully disrupts or takes control then the russian offensive against Kharkiv is finished.
Moreover the Russia northern army will run out of foods, waters, ammo very soon. Rerouting the supply is possible but will cost the russians more time, more money.
This is how the war progresses. Ukraine attacks the russians in their home land.
Well, they could try bolster their front lines instead of leaving them near collapse. Opening a new front whilst you cannot handle the current ones is just insanity.