I think typically as a solution to serious injury (e.g. result of a predator attack or otherwise) that can't be mended.

In my case, we had too many roosters and their competitive/protective behaviour was causing serious injuries to the hens, so we had to make the tough decision to reduce their numbers. Being in the middle of nowhere, there weren't many options for the rooster in question, so it seemed like the most humane thing to do at the time.