Because its a big tactical advantage that they opened a gaping hole in Russian defensive lines.
Defenses are strongest in the direction they are facing. They are very weak from the sides and even worse from the rear. (Ukraine now has a lot of Russian rear it can take advantage of, from Kursk)
If Ukraine has the manpower they could take this little bit of land, manoeuvre around the Russian lines, wreck their shit and get back a lot of land that was stolen from them.