Why we use App_Browsers folder?

The App_Browsers folder inside a website contains site specific browser definition files. These files are XML files that are used to describe characteristics and capabilities of browsers.

When a Request arrives at an ASP.NET application, the ASP.NET run time reads the information in the Request header to determine the client browser and it's capabilities. Now, based upon the browser information [From the browser definition file], ASP.NET server control adapters might generate different XHTML markups so that the final content is rendered correctly on the client agent (browser)

You can find existing browser definitions (machine level) at %SystemRoot%\Microsoft.NET\Framework\[FRAMEWORK VERSION]\Config\Browsers\. 

If you want to change the definition of any of these browser, You can copy the appropreate file to your application's App_Browsers folder and modify the definition as per your requirement.