What purpose does HTML5 serve?

Supports new extra tags and compatible with old Html language. But it needs Html5 enabled browsers to function with new capabilities.

It is to improve HTML applications.It support may new Interesting  features like multimedia support audio and video,new form control attributes like date, time, calender etc and also support the new graphic elements like svg and canvas. 

Html5 added a lot a capabilities over html4 and xhtml. In fact you can make a whole web application or desktop application using html5. It is like a programming language which support many features like GPS, local storage, canvas, SVG, new forms elements, video player, CSS3 box shadow and animation. You can make a responsive website using html5 new tags a like header, footer, article, side, etc.

As mentioned above, we r having many uses in HTML5. HTML5 with CSS3 helps to built responsive site. We can achieve many animations without flash.