This is a great question Pawan.
In the book you will find ideas and tips on when to use web workers in your sites/web apps.
There are many cases where it will be smart to off the 'load' some our main thread. Few examples:
* Complex mathematical calculations for security, webGL etc'.
* Make network requests - Ajax is good but think on cases where you want one point of 'sync' for your web apps.
* Access local storage - local storage is not async action! so the usage of web workers here make a lot of sense.