It exists because I personally wanted to prevent my clients from putting gigantic digital camera photos onto their servers. This plugin is a tool to simply restrict uploaded image sizes to within a defined range. i.e. the original image is 4000×3000, this plugin will limit it to say 1200×1200, discarding the original 10MB file. Then, it’s the job of the theme author to correctly use add_image_size() to create versions of the image suitable for placement within the theme.
Using add_image_size() doesn’t cover what this plugin does as it doesn’t delete the original enormous image.
As for comparison to Imsanity, it looks as though they both do the exactly the same thing. It also seems that Imsanity has been around longer however my original release for this plugin 2 years ago was to fix a previously abandoned a plugin that WordPress 3.2 made faulty due to a change in coding standards. So again, why does it exist? Why not? Why do Toyota cars exist when perfectly good Ford cards that do the same thing exist? Macdonalds and Burger King, PCs and Macs…