Muscle cells can get longer and shorter. how does this ability fit in with their job in the body?

Muscle tissue has a physiological property, contractility, provided by  muscle cells or myocytes, which involves reducing the length of the cell.

The function of skeletal muscle depends on the precise alignment of the myofilaments of actin and myosin in the myofibrilla. This is allowed thanks to  proteins that bind the myofilaments together and gives them the necessary elasticity  allows to regain their initial shape after the contraction (they increase their length).

The contractility of the heart muscle is essential for the survival of the patient. Without contractility, no more rhythm. The contractility of the biceps, triceps and quadriceps. allows to move the different members of the body (depending on their position) thanks to the tendons that are connected with the bones.