A lot of FPS players amp up the dpi on their mouses. The biggest downside to this is while sniping this high dpi can make it almost impossible to keep the crosshairs on the target because even the slightest flick of the wrist on a zoomed in camera will cause the aim to be wildly off. A good way to test this yourself is to take a camera and zoom it out as far as you can and try to take a picture with it; just breathing will make this a challenge because of how much the camera is moving. The "Aim" button on the mouse drops the dpi to a much lower number, usually around 800 or less, which makes lining up those head shots a lot easier when your native dpi is 6400.