As far as I know, these are used for alarming. In the 1500, I think it uses system text lists for the system diagnostics, and you can integrate your own customer text lists into things like the ProgramAlarm. This allows you to manage alarms only in the PLC, and the Comfort Panel doesn't even need to be updated as alarms are added. As a bonus, you can the alarms can dynamically include other text like the FB instance name, so it can automatically generate different text for each instance of the alarm, vs doing it manually for every device in the HMI.
The 1200 doesn't have the ProgramAlarm instruction. I think the only thing you can use them for is Gen_UsrMsg, which allows you to create your own custom entries in the Diagnostic Buffer of the PLC.