The reason is simple - vibration. When a gun fires, the blow from firing vibrates the whole gun like a tuning fork and can vibrate loose any threaded part that is not screwed down tight or staked. Sometimes parts are loose from the factory, but it is also possible that a previous owner was one of those folks who absolutely must remove every tiny part of a gun just for the heckuva it and didn't get things tightened back up again.
You might check www.gunpartscorp.com
for the part, or it can be easily made. Its purpose is to provide a point the slide stop plunger can bear on to keep it down until raised by the magazine follower.