From the plugin site at WordPress:

When you upload an image to your WordPress site, WordPress auto-generates lots of sizes of it. However, there aren’t many good ways to see and use those sizes:

• The Media Attachment page in “Media” doesn’t list anything about the different image sizes, or the permalinks to use them.
• “Add Media” in the post editor doesn’t always show you all the sizes that exist (such as sizes intended for featured images).
• There’s no easy way to simply copy a custom-sized image’s permalink.

Until now! Display All Image Sizes displays information about every size of every image: name, dimensions, and permalink.