In any of the auto-exposure modes, the camera will re-evaluate the exposure for each shot. But there are many situations, such as studio photography, where you know the lighting won’t change and you don’t want the camera continually ‘re-interpreting’ it. If you already know the correct exposure, you can use Manual (M) mode instead. This is quicker than trying to override the camera’s auto mode.
Manual mode puts you in control of the shutter speed and aperture settings, not the camera.