That's actually an easy question.
The answer depends on your answer to this: Are there any workflow automations that yo would like to do that you can't do with other tools?
When do you need to automate a workflow?
Any workflow that is repetitious, or very complex/difficult to perform.
Take a look at the macros located here for some good examples of what KM can do:
As @kcwhat suggested, KM is a broad-based Mac automation tool.
It is like having a master mechanics tool chest with thousands of tools.
Having said that, every mechanic knows that sometimes you need a specialty tool to do a very special job.
So, if I were starting from scratch, I would first buy KM.
That's going to solve 95-99% of my automation needs.
Then, if a special use case arises, I can decide if it is worth it to buy a specialty tool.
The additional automation tools I have bought are:
- LaunchBar
- Script Debugger 7 -- IDE for AppleScript