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In ASP.NET 4.5, support for asynchronous programming has been added not just to the language but also to the MVC, Web Forms, and Web API frameworks. For example, an ASP.NET MVC controller action method receives data from a web request and passes the data to a view, which then creates the HTML to be sent to the browser. Frequently, the action method needs to get data from a database or web service to display it in a webpage or to save data entered in a webpage. In those scenarios it's easy to make the action method asynchronous: instead of returning an ActionResult object, you return Task and mark the method with the async keyword. Inside the method, when a line of code kicks off an operation that involves wait time, you mark it with the await keyword.

Under the covers the compiler generates the appropriate asynchronous code. When the application makes the call to FindTaskByIdAsync, ASP.NET makes the FindTask request and then unwinds the worker thread and makes it available to process another request. When the FindTask request is done, a thread is restarted to continue processing the code that comes after that call. During the interim, between when the FindTask request is initiated and when the data is returned, you have a thread available to do useful work which otherwise would be tied up waiting for the response.

There is some overhead for asynchronous code, but under low load conditions, that overhead is negligible, while under high load conditions you’re able to process requests that otherwise would be held up waiting for available threads.

It has been possible to do this kind of asynchronous programming since ASP.NET 1.1, but it was difficult to write, prone to error, and difficult to debug. Now that the coding for it is simplified in ASP.NET 4.5, there's no reason anymore not to do it.

Source of Information : Building Cloud Apps With Microsoft Azure