How fast your website loads is one of the signals which many search engines, specially Google takes into account when ranking you in a search result. Though it might not be just applicable for every scenario but if you just take User Experience in mind, let him browse as fast as he can and get done what he came for is an important factor.
The site is still in beta stage so it seems there is no explanation or comparison give but when it gets into a final stage you will have option for uploading a file, or use a url of the file and it will also store all your previous downloads.
Now if you are in doubt I will suggest to try PageSpeed tool from Google which has a Firefox, Chrome and an Online Version with optimization for Mobile and Desktop version. When Page Speed is used on your Desktop you get option to download optimized images and scripts.