For application and session event handling we need this file. The Global.asax file is optional. You create it only if you want to handle application or session events. If you do not define the file, the ASP.NET page framework assumes that you have not defined any application or session event handlers.
Refer the links for details:
MSDN : Global.asax File[
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MSDN : Global.asax Syntax[
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--Amit