[LAC-Tech]

Idempotency (as I understand it, what maths people might call an 'idempotent function') is a very core idea in distributed systems - networks are unreliable, stuff may get lost, or you might not get an acknowledgment back, so the ability to send the same thing 1 0r a million times and end up with the same state is useful.

Or have I completely missed the point of your question..?

[NortySpock]

It is also super useful in ye olde batch process / etl process. Designing an ingest-analyze-report process to checkpoint its work and recover gracefully even when started at an unexpected time or place means you can retry safely rather than have to manually clear out the detritus of a partial job run.