Exposure compensation often serves two purposes in phones. I don't have the S9, so I'm just giving my best educated guess.
In Pro (aka manual) mode, you are likely able to either manually set the ISO and shutter settings or let the camera do it. Exposure compensation only compensates the camera's proper exposure algorithm when letting it set ISO and shutter speed itself. Normally, the camera tries to expose to a neutral brightness. Adjusting the exposure compensation in this "semi-auto" mode tells the camera to over or under expose the image.
When you go into full manual, the camera is not making any of the exposure decisions, thus there is nothing to compensate for. YOU are the exposure compensation, so to speak, by adjusting the ISO and shutter yourself. Since the exposure compensation serves no purpose in full manual, it becomes an exposure meter. It tells you if you are over, under, or correctly exposed based on your settings. This acts as a guide to help you judge your settings before you take the shot.
To test this, open your app in Pro mode and see what the exposure compensation/meter button reads. Then adjust your shutter speed up and down. As you move to a faster speed, you'll see the exposure meter move to a lower/negative number and the live preview will darken accordingly as it becomes more under exposed. Vice versa if you slow down the shutter speed, and your ISO affects this as well. Moving between bright and dark scenes will also cause the meter to fluctuate if you don't adjust the ISO and shutter speeds accordingly.