Even a refurb "should" come with the System Disks. The Disk Utility that comes in the Applications folder will not repair the Disk that is running the System (that would run the risk of trying to overwrite a part of the system that was in use) so you use the first install Disk to run Disk Utility on a minimum system from the CD/DVD. That way you can repair a part of the internal drive directory if you need to.
Having said that I bought a repaired G4 recently (cheap) that didn't have the original disks, but the shop burned me a DVD of the Instal disks I needed. They suggested not running Leopard on a G4 though people do ..