Do you mean the activate/deactivate action?
One example -- check the "Last Modified" of ~/Library/Application Support/Keyboard Maestro/Keyboard Maestro Macros.plist, wait a minute, disable a Macro Group, check modification date again. Then re-enable the Group and do the same but with a the "Activate/Deactivate" action. Re-writing the entire Macros.plist isn't so bad in these days of SSDs, but we weren't always so fortunate 
More sensibly -- the "Activate/Deactivate" action overrides the Group's activation criterion. So if you have a Group that is set to be only available for one action, because that's your main use-case, you can use the "A/D" action to make it available for multiple uses then deactivate it again. Or if the Group has been previously deactivated you can programmatically activate it for one action.
I don't use it myself, so I'm clutching at straws as to how it could be used. And "how can I use this action?" is straying even further from the thread topic than we'd gone before. If you want proper answers then post that question in a new thread so people will see it.