One of the most common uses of layers in Adobe Photoshop is to combine multiple images in a single document. In its simplest form, this is just adding related photos to a new file so each picture is visible. Having the individual images on separate layers allows you to rearrange them on the page easily and apply a correction to one but not the others. In this example, I made the bottom right photo a sepia tint to hide the red Christmas tree skirt the cat is sitting on. This change prevents the red from drawing the viewer's eye away from the other shots.
Learn more ways to employ layers in Using Layers and Masks in Adobe Photoshop, an online course that begins October 5, 2020, from the Lifelong Learning Center in Missoula, Montana. Click here to register.