I've only written custom workflows so far. We use them to proceduralise tasks that need to be repeated a lot. We did this to remove the "human factor" and make common tasks more repeatable.
The last workflow I wrote was for replacing EOL kit. It backs up current settings (like address, name, notes, etc), removes the EOL device from the network and gives it a special name for tracking and rollback, adds the replacement device, and restores all the settings.
Previously, one engineer would be scheduled to replace one EOL device at a time.
Once this process was implemented, we increased that to 6 concurrent replacements.
This allowed each engineer to complete 6x more of these tasks in the same amount of time. What was a 6-month rollout became a single-month rollout.