It is just a manufacturing shortcut. There is a spring inside the recoil spring guide that powers that ball in the back, and the ball in turn acts to push down and also help retain the slide stop. (The magazine follower pushes it up when the magazine is empty.)
Originally, a cylindrical plug screwed into the front end of the guide to keep the spring in place. But that involved machining and threading the plug and threading the guide, as well as cutting a slot in the plug. Around 1981, they changed to a simple ball bearing that was held in place by swaging over the end of the guide. Not many people ever noticed the difference.