I think the title says it all. ‘?’ just opens the window, further clicks do nothing. It’s better that the window can also be closed by it.
The question mark is a pushbutton while the “External editor” is a toggle button.
I doubt this was intensionally but for instance the VSampler 3 plugin causes a horrible GUI crash (which is safely captured by Renoise) when clicking on the close icon of the plugin frame itself, (where the External editor button performs a window close without any error report)