CE Gravatar is an ExpressionEngine® plugin. This means that it does not have a user interface, but is intended as a development tool to be used in your templates. It allows you, as the developer, to dynamically include Gravatar images on-the-fly in your templates.
Because it provides an easy interface to all of the options currently available for Gravatar images. You can even run conditional statements to check if a Gravatar image exists for an email address. A better question is, “Why not CE Gravatar?”