Adding a Post Crop Vignette, which subtly darkens the corners of your image, is a popular way of putting a finishing touch to your image and drawing attention to the subject. This technique, using Lightroom’s Effects Panel in the Develop Module, is different from using the Radial Tool. The main difference is the Post Crop Vignette effect adds it to the entire image, while with the Radial Tool you could lighten or darken a specific area of your photo, i.e. add an off-center vignette. You can learn more about how that works in this video. Both have their use depending on what you are trying to accomplish.