Whyshouldwe use thepostback?mostly where should use and uses

First, let's see what happens when you browse a web page of a site. 

Postback is a property of the web page in ASP.NET to indicate whether page is being rendered for the first time or is being loaded in response to a postback.

You could check the value by writing Page.IsPostBack or just IsPostBack.

When the page is requested for first time, this property returns false in the page load event and on successive requests returns true as value.

For eg., when you click a button which has server-side event handler, then the page is posted again and now on the page load, this property will return true as value.

In a typical scenario, while building web applications using ASP.NET, you could do the initialization work required for the aspx page. For eg., fetching the lookup data for dropdown controls, fetching the page level data and binding to input controls etc.

Hope this answer helps you.