Sometimes you want to deliberately over or underexpose a scene because you know you want to have more details in highlights/shadows and plan to pull the footage up/down in post anyway.
But that means that you do not see what you will actually end up with on the preview monitor during shooting.
Here the Exposure compensation feature comes in as it will do the pulling up/down already with the preview video stream so you get a good estimate of what data ranges you will actually end up with in the end in post during shooting already.