If you have a lot of data, keeping backups for too long can be costly. But a lot of datasets are small (e.g. few GB) which makes keeping backups for really long time cheap enough.

If you're datastore is small (e.g. SQL table), but supersensitive (E.g. financial data), you may want to keep backups for really long and on medium that doesn't allow easy modifications (e.g. tapes or AWS Glacier).

I know some government (e.g. Poland social security) keep several backups in many locations for many years just as a redundancy.