In real time systems gdg's are used for storing
1) day to day
2) month to date
3) year to data .
Cases may arise where we need to go back and find out what was the data that was processed or u r company finance dept would have some money issues which they would like to settle by going back and finding out how many contract's/policies were sold .In such cases this back up data can be used.
again the requiremnt could change according to u r company's requiremnt.
This was just an example.