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.
Give IUSR permissions on your wp-content folder and IIS_IUSRS permissions on your Windows temp folder. Note: If you have edited your php.ini file and change the upload temp directory then you will need to give IIS_IUSRS group read permissions on that folder instead.