Ukraine struggles to repel the constant attacks on the eastern front so by attacking Russia in the north, it forces them to move troops from elsewhere thus making them to stop the attacks due to lack of manpower and go on defence instead. This eases the pressure on the Ukrainian defenders.

This newly acquired land is also something they can use to trade back parts of their own land if and when the peace negotitations start.

It also further diminishes the credibility of the Russian army and leadership.