alt Hacker NewsAuthor here. That still doesn't get at my confusion over the UX. It wouldn't change any of the memory management issues to have a double-click on an app icon jump straight into the application. That user intent seems pretty clear to me, so the extra step could have been automated away. Maybe RISC OS users just had a different way of working?
If you double clicked on a file it would load the associated application and open the file.
The only reason I can think of is to not disrupt the user's flow by opening a window on top of the Filer windows. Maybe they intend to open multiple applications to use together.
There was a carefully written programmer's guide for UX. That might have an explanation.