Joints are formed where two or more bones meet. We need joints that permit little to no movement in our skeletons to keep us upright. These joints are known as structural joints and are held together by fibrous connective tissue. Simply stated, if all our joints are able to move as our functional joints do, we will end up all floppy and jelly-like. We won't be able to keep our bodies upright or move as we currently do.
