Air cover is really crucial for a counter-offensive, because basically if you are exposed to Russian air attacks, your mass tanks and troops are going to become easy targets. Ukraine needs this air defense capability to effectively protect its guys on the ground who are trying to make advances. Russia has done through these quite regular bombing raids is it's used up a lot of Ukraine's surface-to-air missiles so that they don't have a huge amount left to take down fighter jets and bombers If and when Russia starts to use those.