Following is a list of just some of the features which include in a web page.

Animations  -Some animation can be achieved with CSS transitions, but for true interactivity, you need to control graphics directly using Javascript.

Geolocation - To provide location-aware services to your users, you must access the data using Javascript.

Canvas - The only way to draw on the HTML5 canvas is with Javascript commands. 

Audio and Video - You can add these with simple HTML5 tags, but to control playback directly, you will need Javascript. 

Form processing  - If you wish to make your forms easier to use by having in-browser validation, user prompting, and error checking, then you have to do much of this with JavaScript, although HTML5 is making some great inroads in this direction.

Ajax - Any behind-the-scenes communication with a web server you need to make has to be done via JavaScript calls.

Dynamic Elements - Some elements can be changed via CSS and mouse-overs, but for anything more, such as resizing, moving, or dragging and dropping, you have to use JavaScript.

In-browser Gaming - Games that run in the browser without plug-ins such as Flash are written in JavaScript.

Floating Elements - Pop-ups and other floating elements that follow the page as you scroll are all implemented with JavaScript. 

Hover Effects - Thumbnail or advanced tooltips that hover over HTML elements are created and controlled with JavaScript.