When shooting with sights, you hold the bow straight up and down, so the sights will work. When shooting instinctively, you don't have to do that. Most of us have a natural cant, or angle that the bow is held at, that is more comfortable than the straight up and down position. My natural cant is where the bow is about 5-10 degrees off vertical. You have to make sure that your string hand matches the cant, so you don't torque the string. Usually you do this by holding the bow loosely so that the angle of the bow will naturally correct for the angle of your string fingers.
Other reasons for canting are to get the bow out of the way so you have a better view of the target, or to hold the bow at an angle while hunting that allows for clearance of brush or obstructions. You can learn to shoot well while holding the bow at a variety of different angles.