Imagus is a very convenient extention because it allows users to view thumbnails, reddit posts, image links, gifs, and videos just by hovering the mouse over these things. They also enlarge images for better view. It especially works well with links to the site imgur, allowing you to scroll through images without opening the entire website.
Edit: I should also add that it works very efficiently and seamlessly. Instead of having to load adds or other features when visiting a website, imagus removes that process and only has to load the image or video, making a noticeably faster load.