Ensure uploaded images don't take too much disk space! Most of the time, you don't need that 6000 x 4000px image, as you're using transforms on it in your templates (if you're not, you should). Image Resizer takes care of this for you automatically, resizing images to a more sane default.
Resizing can occur automatically during upload, or on-demand through the Assets index in Craft. Setting a maximum possible width and height in pixels, Image Resizer will check images being uploaded that they are under these limits. If over, it'll be resized.
Image Resizer is also smart enough to keep the aspect ratio of images when resizing.
Each time an image is resized, a log entry is created to provide feedback on the task that has run. This is useful for debugging issues with specific images, but also provides a way to see how much Image Resizer has saved from resizing images.