Implementation of IsPostBack in page load

In short, you use it everytime you need to execute something ONLY on first load.

The classic usage of Page.IsPostBack is data binding / control initialization.

if(!Page.IsPostBack){   //Control Initialization   //Databinding}

Things that are persisted on ViewState and ControlState don't need to be recreated on every postback so you check for this condition in order to avoid executing unnecessary code.

Another classic usage is getting and processing Querystring parameters. You don't need to do that on postback.

When there is no need to repeat the operation other than the first time.

use it with expensive operations (such as getting data from a database or populating ListItems) that must be performed only the first time the page or control is loaded. If the page is posted to the server and then reloaded, there is no need to repeat the operation. By testing the value of IsPostBack, you can skip the expensive operation,

It's for processing form data.

If you want to handle POSTed data, you only want to do so if the page actually posted data, not on first load. Hence, the IsPostBack flag.