HOLY COW. I've been using Method for several years and I'm JUST NOW SEEING THIS!?!?!?!
Apparentlly Button Actions allow you to not only copy Action steps from elsewhere on the current screen, you can also copy action steps from ANYWHERE else within your app/account.
For me, the initial benefit here is I can write actions in one place and if I ever need the same logic elsewhere, I can copy it rather than duplicating it manually. While that's a nice benefit, that also can lead to unwanted code/action duplication; so if you change the script logic in one place, you have to make the same change in any other place(s) you copied the script. That's not good.
So...knowing I have this ability to copy entire actions from anywhere in the app, once I do need an action to be available in more than one place, I could first move the SOURCE action onto it's own screen where I would maintain and make any changes going forward to the action on that screen. Then, when it's working and it's ready to publish, I could copy the action to the screens that use it.
I really wish (feature request) Method allowed me to EXECUTE these actions from anywhere in the app (like a global function) so as to avoid the rigamarole of copying action scripts around the app. But if this is how I can avoid duplication of complex actions in Method, then so be it.
This is a big time hack using Method in this manner and basically a poor man's refactoring capability (for those that have written custom software before, you know how important easy refactorings are). One other side benefit: on the screen where the source action will be housed, you can write small test harnesses on the screen - using text boxes, messages, etc - as you develop/maintain the action logic to help ensure it's working as desired. Once you have it working the way you want, then copy the script ("publish") to the screens where it's needed (again, if you've developed quality software before, you know how important developer testing is).
Anyway, I hope all this makes sense and hopefully others will somehow find this information useful.