What’s the use of "GLOBAL.ASAX" file?

Answer:

Global.asax is a file that resides in the root directory of your application. It is inaccessible over the web but is used by the ASP.NET application if it is there. It is a collection of event handlers that you can use to change and set settings in your site. The events can come from one of two places - The HTTPApplication object and any HTTPModule object that is specified in web.confg or machine.config.