This module should be in the core... a long time ago. If you use the CKEditor to resize images, the module filters away the pixels that are no longer necessary. The original picture and a smaller copy is stored in your folders. Your page will show up faster. Good for the SEO too..
Every site wants a good search engine ranking. The metatag modules is needed for that as well as this module. A sitemap is een collection of the nodes you want to be indexed by the search engines. Usually this sitemap is on the front page. Search engines find them en start indexing.
This module allows the administrator to redirect a URL to an other, in case the first one does not exist any more. This can happen if you moved your content to an other place, renamed the URL, or if you have a new site with almost the same content..