You want the monitor frequency higher than the game FPS. So game at 180fps and monitor at 240hz is perfect. Then enable Freesync and the monitor with automatically adjust to match the games FPS for super smooth motion.
If you had the monitor at 144hz and game at 180fps you get screen tearing. You could use VSync to reduce the FPS to 144 to reduce tearing but that increases input lag. This is why you use Freesync. The monitor will auto adjust to match the FPS of the game to reduce problems with tearing, blur, missed frames, etc....
If you happen to ever play a game where the FPS are reaching values over 240hz this can cause problems too. The solution is to limit the FPS to 240. One way is through VSync which increases input...