Adjustment layers allow you to make changes to a photograph like exposure, colour, contrast, brightness etc without permanently changing the photography. It also means you can try different adjustment layers and turn them on and off to compare which one works best. So you can have a layer that applies sepia and another one for black and white, by showing and hiding the different layers you can see which one works. You can do that same for other adjustment types too.
Here is a video to show you how it works.