Angle Snapping makes tracking on straight lines more straight. An easy way to test this is using MS Paint and drawing straight lines, you will see that it is easier when Angle Snapping is turned on.
However, we do not recommend using it in games that require tracking skills, like FPS games. The reason is that when it is turned on, the tracking and mouse inputs are recalculated, and will not match the real movement that you are making with your hand.
To put in perspective, if you are aiming and moving horizontally on a straight line and you suddenly make a micro movement vertically, it may not be recognized, and you may miss your target.
Instead, we recommend that you practice drawing straight lines without it, train your muscle memory, and adjust other settings such as DPI or sensitivity until you find the most comfortable settings.