The Static plugin provides three useful functions.
Firstly, it takes on the task of serving up files to the user. Several other plugins rely on this aspect of Static rather than handling simple file reading on it's own. Why? Because it's much easier to maintain and extend this if it's done in one central place, and there is no reason to, for example, guess MIME types in 10 different ways across 10 different plugins.
Secondly, it lets authorized users upload files. Pictures, code snippets, spreadsheets and keyfiles. It doesn't discriminate at all, it just accepts the file, stores it and serves it back to any user.
Thirdly, it provides a handy list of uploaded and built-in files so that they may be easily included elsewhere.