When WebReports is installed, two users called "appuser" are created. One is a local Windows user, and one is a SQL user.
These users are or have been needed by WebReports
The local Windows user account ([PC_NAME]\appuser) is used for importing report packs (RDL files) and synchronizing between WebReports and Microsoft Reporting Services. For more information about this account, refer to article #13598. The plan is to remove the need for this use in a future version. In order to change the password for this Windows user, refer to article #17815.
The SQL user account has been removed from 1.8 and later versions as it's not essential for the Web Reports to operate correctly.