You may wish to expose server side configuration via JavaScript. For example, you may have a different base url for your REST api per environment (dev,test,uat,prod) which your JavaScript AJAX calls need to consider.
Typically, you would push the configuration data down via an html5 data- attribute, a hidden input field or via explicit assignment to a JavaScript global. You usually end up doing this for every item you wish to expose. However, using a custom MVC ActionFilterAttribute and a Server-to-JSON serialization technique you can easily expose your web.config appSettings for controller actions.
NOTE: Exposing the entire appSettings configuration section may expose your application to security risks.