HTML 5: Why you need HTML 5 ?

There are many of blog and article the explain, in-details, why you should use html5.
I'll give you the link on the end of this article.

Before answering the the question of this article title, let me share what it HTML5.

HTML5 just new HTML specification on top of previous version of specification.
If you have knowledge and experience on html 4, that enough to moving forward.
It come with some new tag that mainly to offer more rich of web application development.

So, why you need HTML 5?

1. It bring new tag with rich content capability. You can have audio, video content with our 3rd parties plug-in need to installed (use <audio> and <video> tag). even it is possible to built game application using <canvas> tag 
2. Code is cleaner. instead of using <div> tag with id then manipulate the content using javascript, html5 come with specific tag. <section>, <article>, <header>, <footer>, <aside> and <nav> tags will give you more meaningful tags. 
3. It is mobile ready. Mostly no need special converter code to display web application on the desktop browser or mobile. it just ready!
4. No more painful with 3rd parties plug-in. use pure html which will support by cross-platform browser. include geo-location API.

Here some more details article for future reference. 

http://tympanus.net/codrops/2011/11/24/top-10-reasons-to-use-html5-right-now/
http://smallbiztrends.com/2014/02/what-is-html5.html
http://www.cognifide.com/blogs/ux/5-reasons-why-html5-matters/