Anywhere you need something that is not dependent upon a third party app is probably the most obvious one.
In terms of ease of use, certainly for many people starting out in automating their Mac, there are a good number of examples of how to build simple standard Automator workflows that there simply are not examples of the equivalent for, for Keyboard Maestro. That isn’t to say they can’t be done.
Also for many starting users, Automator is like a small toolbox of tools, whereas Keyboard Maestro is more like an industrial workshop. It can be overwhelming and intimidating.