Muscles are a critical part of the musculoskeletal system and play a vital role in almost all systems and functions of the body. Different muscles have different functions. Together, they are responsible for: 

• Pumping blood to and from the heart and blood vessels 
• Movement, posture, stability, and balance
• Breathing 
• Joint flexibility and range of motion
• Swallowing
• Food digestion and elimination of waste
• Protection of bones and organs inside the body
• Facial expressions
• Speech 
• Urinary and bowel control
• Seeing and hearing

In summary, muscles are essential for almost every aspect of human function and movement, from basic actions like standing and walking to more complex activities like speaking and playing sports. They are integral to overall health and well-being.