Perhaps because the next option under "Screen" allows you to get more specific than the entire screen. You set it to "all screens" which probably isn't a useful option.
However now that you mention it, your action above could be very useful if you just wanted to flash all the screens "green" to get the user's attention. (Just check the Display checkbox.)
I still don't quite understand. Even if you set it to click within a certain area, what's the point?
I think you misunderstand. That "area" you refer to is absolutely NOT necessarily the area where the mouse will click. It's the area of the "source image" that the action will use to find the image elsewhere on "all screens" as you indicated at the bottom of your action.
In your case, the source image is within "all screens" so it will probably be the "best" match, as you indicated, but if you switch "best" to "rightmost" you could get an entirely different match.
Here's a practical example. Let's say you were trying to automate a computer game. The game displayed (in the upper left corner of your screen) an image of a target you are supposed to click on to win the game. And perhaps the image keeps changing, to keep the game interesting. In this case, you could easily solve this problem with this KM action. The image you would search for would be taken directly from the screen using the "Screen"/"Area" options, and then you would specify the target area in the bottom of your macro to be a different "Area" of the screen. What a simple solution to a complex problem!
That makes a lot more sense. Thank you!
There are so many toggles in the Move or Click Mouse action, (perhaps more than any other KM action) it can be difficult to fathom all the options. Not all the combinations are useful, but that doesn't mean they should be removed.