It is kind of a first step when writing software - do I want GUI interface windows, or am I going to do all that myself?
It was a big thing to turn off the OS and do all the work yourself. If the OS was going to be a hindrance for what you were trying to program, you turned it off, either right at the beginning, or selectively in the main body of code for events that needed a little privacy and complete control of resources. The snag with the latter method, you got a glitch when handing back control and telling the GUI to clear the screen and redraw everything (this is observable behaviour with some software).
