Actually webadmin has three types of authentication: "browser authenticaton", "repo login" and "system manager login", which all have detailed information in the webadmin pdf help file under installation package .
And we do not support anonymous authentication in IIS because for anonymous auth in IIS the user profiles will be the same.

I am not sure which situation you are in, I suppose the two situations:
1. web authentication is annoying, and you want to skip it
   enabling windows account is for browser authentication so that IIS can get different user profiles, and this step can not be skipped.
   By default browser settings:Chrome and IE will auto login using your current windows account for browser authentication, Firefox will let you manully login for browser authentication.
   "browser authentication" has nothing to do with "system manager login", login as system administrator can give you the right to do more server level operations, if you just want to do actions in repo, you can straightly login the repo without system manager login.
  
2. other people who do not have the domain windows account want to visit webadmin in domain
   you can use local account instead of domain windows account to login browser for browser authentication.

the links below may help you:
https://msdn.microsoft.com/en-us/library/aa292114%28v=vs.71%29.aspx (http://support.microsoft.com/en-us/kb/102716)
http://manual.aspdotnetstorefront.com/p-1614-enabling-windows-authentication-in-iis7.aspx