There are already many other plugins to handle fields, content and data, so why ItemManager ?
What sets ItemManager apart is that it lets you quickly setup new content and functionality by using it's own api and xml database. This makes it very universal and portable. You can use it in new projects or embed it in already existing projects. See more about the api reference here.
ItemManager can save you from searching third party plugins or reinventing wheels to get things done. It can be used for websites, webshops, blogs, galleries, and also for various data storage, structures, settings, tools, custom plugins, etc.
Inside GS as a plugin ItemManager works as an extended backend and lets you edit directly your setup.