Mobility vs. Stability.
Some joints are made for stability, others for mobility. According to Gray Cook, an orthopedic physical therapy specialist, “Stability is the active muscular control exerted on a joint to redirect force and controlled movement in the presence of normal muscular flexibility and joint mobility.”
Starting with your feet, joint function alternates like this:

• Ankles: mobility
  
  
• Knees: stability
  
  
• Hips: mobility
  
  
• Lower back: stability
  
  
• Middle back: mobility
  
  
• Shoulder-blades: stability
  
  
• Shoulders: mobility
  
  

Your body likes balance. If one joint lacks mobility, other joints will compensate. Problem is joints have their function. A mobile joint is not made for stability & vice versa. This helps understanding injuries.

Example: Your knee hurts. Look at the joint below & above: ankle & hip. If one of them lacks mobility - they’re stiff - your knee will compensate by being mobile. But your knee is designed for stability not mobility, so it gets hurt.