Java is a programming language and computing platform first released by Sun Microsystems in 1995. There are lots of applications and websites that will not work unless you have Java installed, and more are created every day.
IIS is able to impersonate the IUSR_<computername> account because the user name and password for this account are known by IIS. Before returning a page to the client, IIS checks NTFS file and directory permissions to see if the IUSR_<computername> account is allowed access to the file.
Unless the IUSR account is given access to the Inetpub site directories specifically, or the Users group is allowed access, the websites return a 401.3-Unauthorized error. I added the IUSR account to the IIS_IUSRS group, which I would prefer to use, and added that group to the permissions of Inetpub directories.