When you insert images in WordPress, it’s always a specific image size. It can be an image size such as Medium, Large or something else set by your theme or another plugin. When you update your media sizes settings (or switch theme), you need to regenerate those images (with Regenerate Thumbnails, for example). But what about your posts content? They will be still using the old images, which aren’t actually part of the new metadata, and little by little your WordPress will become broken. This plugin fixes this by regenerating the thumbnails/images in your content and keeps everything clean. Once this is done, it’s a good idea to run a tool like Media Cleaner on your install to detect the useless files.