Ligaments govern a joint's range of motion. As a connector between joints and muscles, ligaments keep muscles from stretching a joint so far that it becomes dislocated. Often, several ligaments guide one joint. This is the case for spinal ligaments and vertebrae, for example.
Ligaments also provide an important function in stabilizing joints and helping them to work normally for the body. Without ligaments, muscles could pull too hard, or in the wrong direction, and the body would lose substantial dexterity. For example, the anterior cruciate ligament (or ACL), which is a ligament behind the knee, helps make sure the knee is moving in the right direction and evenly when the leg muscles pull. Because ligaments are not very elastic, the ACL is frequently injured in high-impact sports such as football, when the knee is struck, twisted, or otherwise loses stability.
Bones are susceptible to damage when you move around, but ligaments are there to move around and absorb impact instead. For example, they will protect a joint like your elbow joint when you are trying to pick something up and the weight of the object threatens to damage the bone and muscle.