It has the minimum permissions and user privileges those are necessary to start and run a worker process on IIS.
It is necessary that Application pool identities be members of this group so that the application pool can register with Http.sys.
The IIS IUSR_ComputerName user account is for anonymous access to IIS.
By default, when a user accesses a Web site that uses Anonymous authentication, that user is mapped to the IUSR_ComputerName account.
The IIS IWAM_ComputerName user account is for starting out-of-process applications in IIS 5.0 isolation mode.
The built-in ASPNET user account is for running the ASP.NET worker process in IIS 5.0 isolation mode.