Utility items are crucial for success in CS:GO, using them effectively can put your team in a position to win. Smokes, flashes, grenades, and incendiaries can block off territory, disrupt an area, and eliminate the enemy. If your Terrorist team on Mirage is pushing down Catwalk to the B site, well-timed smokes for Window and Connector will make that trek far less risky.
Then, if you flash out Bench and Market as well, your team should be able to plant the bomb and set up for defense. Strategies that make use of utility are incredibly difficult to stop and can win a round much easier than rushing or playing spread out.