Vignettes do a lot for a photo, so learning when and how to add them appropriately is key! You can see in the photo above the corners are darker than the center. This is called a "vignette" and they draw a viewer's eye to the center of the photo. It can focus attention to your subject, in the case of a portrait for example. It also gives your photos a vintage feeling. Using your photo editing software of choice, you should experiment with adding in vignettes to your photos!