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.
You can usually delete these accounts. When you install Internet Information Services, you may find several new accounts, including IUSR_host and IWAM_host, where host is the computer name. The IUSR_host account is the built-in account for anonymous access to Internet Information Services.