When building a web application, we must decide how and when the browser will communicate with the web server. The ASP.NET WebForms model greatly simplifies web development by providing a straightforward mechanism for exchanging data between the browser and the server. With WebForms, each ASP.NET page's rendered output includes a <form> element that performs a postback to the same page whenever a button control within the form is clicked, or whenever the user modifies a control whose AutoPostBack property is set to True. On postback, the server sends the entire contents of the web page back to the browser, which then displays this new content. With WebForms we don't need to spend much time or effort thinking about how or when the browser will communicate with the server or how that returned information will be processed by the browser. It just works.
While this approach certainly works and has its advantages, it's not without its drawbacks. The primary concern with postback forms is that they require a large amount of information to be exchanged between the browser and the server. Specifically, the browser sends back all of its form fields (including hidden ones, like view state, which may be quite large) and then the server sends back the entire contents of the web page. Granted, there are scenarios where this large quantity of data needs to be exchanged, but in many cases we can use techniques that exchange much less information. However, these techniques necessitate spending more time and effort thinking about how and when to have the browser communicate with the server and intelligently deciding on what information needs to be exchanged.