In Firefox all standalone image pages (for example, when you right-click and choose "View Image"), show their image on a white background. This is good for images that were created with the expectation that they would be viewed on a white background.
However, most people who take care with the images that they create don't make this silly mistake. With this add-on installed, you won't have to see that ugly white background behind your transparent images. Now your images will be rendered against the textured dark grey background, making it easier to see where the transparency starts and begins.