If you have a very high contrast image for example half your scene is in the shade and the other half in direct sunlight, the camera will not be able to expose correctly for both so you either end up getting both areas over and under exposed or the camera might expose for the dark shadow area which means your bright part of the picture is then we'll over exposed, or vice versa.
By touching the area on the tablet which is important to you, the camera will adjust the exposure for that area and ignores the rest of the picture rather than take an average of the whole of the picture.
You can test this if you are shooting a sunset, you either have to expose for the sky or the foreground, you can do both. In this case you would touch the sky so you expose for this, letting the foreground go dark.