Manual mode does not seem to provide shutter-priority exposure mode.
If I need a shutter speed of 1/500 sec to avoid blurring with a moving subject (maybe a sports event), I can select that shutter speed in Manual. But the camera will not automatically select the appropriate ISO setting for me, to give a proper exposure.
It can pick BOTH shutter speed and ISO automatically, but if I want to set one of those two values, it cannot automatically adjust the other one for me, to maintain a properly exposed picture. It's all or nothing.