Every OWIN application has a startup class where you specify components for the application pipeline.
If you start a new Visual Studio project, you’ll see pieces of OWIN in it.
OWIN is a specification that defines an API for framework and servers to cooperation.
The point of OWIN is to decouple server and application.
For example, ASP.NET Identity uses OWIN security, SignalR self hosting uses OWIN hosting, and etc., the examples all use OWIN,
therefore they all need to have a startup class, that is defined in “Startup.cs” file.
The Global.asax, the ASP.NET application file, is an optional file that contains code for responding
to application-level events raised by ASP.NET or by HttpModules.
For more details:
OWIN
http://www.asp.net/aspnet/overview/owin-and-katana
Global.asax
http://msdn.microsoft.com/en-us/library/1xaas8a2(v=vs.71).aspx
You can find more ideas about why OWIN in the following article:
http://www.asp.net/aspnet/overview/owin-and-katana/an-overview-of-project-katana