The system state backups include, but aren't limited to, COM + class registration database, registry, boot files, system files and performance counter. It's all dependent on your system. System files vary by OS and service packs. Usually there are several thousand of them. MS Windows makes a dynamic list of these DLLs when you include them in the backup. Including the system files allows you to recover from corrupted system files, or if you accidentally uninstall some service packs, or want to recover with a bare metal restore. It allows you to return to the state of the backup without having to reinstall the OS from the installation kit, and then installing each service pack separately.
Note: No user data file is included in system state backup. A system state backup job should be configured as a standalone job. There should be no other data source included in a system state backup job. \r