Take a look at the ligaments next time you cut into a raw chicken joint. It is very tough material. If the chicken, or you, didn't have the ligament material holding your bones together, at night when you sleep and your muscles are relaxed your bones would get mixed up in a terrible jumble. The nose bone could be down by your waist and your knee cap could be dangling from an ear. You get the idea.
Muscles have the job of keeping the bones together at joints during the day.
Sometimes the muscles do not do their job. Well, since they are voluntary muscles, it might be more accurate to say that you don't make your muscles keep the bones together at the joints. Let's do an experiment. Hold your arm out in front of you with the palm down. Let the wrist flop down on the stop.