Ive done a research on the web and it says that it is used it to keep people from stealing your content. They will probably have copy and paste disabled too. Theres still ways of seeing it. but i dont know how they do it
Ultimately there is nothing you can do to hide any source code that is served to the browser. Any attempts to hide that can be bypassed by simply using an HTTP viewer or other tool that reads directly from the webserver bypassing a browser (e.g., the HTTP viewer at http://www.rexswain.com/httpview.html).
Note that code that is executed on the server, such as PHP, ASP, .NET, JSP, etc are NOT readable, as long as your server is configured to execute those files rather than return the source. Of course, if your server is not configured correctly, then a hacker might get into these. That is a whole other area -- making sure your server is safe from hackers.