Below is a histogram from a typical low contrast image that shows there is no image data at either the top or bottom of the tonal range.
Brightening such an image with a conventional slider control will simply shift the picture detail to the right. In the following image it has been overdone. A good proportion of the image that was merely bright becomes pure white, with an enormous spike at the right hand edge of the graph, indicating that much of the image detail is lost.
Because such a brightening technique also leaves a long flat line to the left of the histogram, it means that the image will appear very "flat" with no dark points in the image. This is why you will normally have to use the contrast slider after using the brightness slider to counteract this effect.
Whereas the Brightness control shifts the image detail to the left or right, the Contrast control stretches it out evenly in both directions, as shown in the third diagram.
However, with an image that has already been adjusted perfectly for brightness, this simply means that still more of the detail of the brighter part of the picture will be pushed off the edge, turning pure white.
With an image that is not as well balanced as was this one before it was brightened, with equal sized gaps on the dark and light sides, you cannot help but lose data at one end of the tonal range or the other.
Unlike the crude Brightness and Contrast controls the Levels Dialogue allows the dark and light end of the tonal range to be adjusted individually, so even unbalanced images can have dark and bright contrast adjusted optimally. And, as a brightness control, the central slider (Gamma control) can adjust the mid-grey point compressing either the light or darker into a smaller tonal range, again without any loss of image data. This is why, with photographs, you should never use brightness or contrast controls. Always use Levels or Curves. (Curves is an alternative, and in some ways still more powerful tool than Levels.)