What is the purpose of the "Z" line in a myofibril?

The portion of the myofibril between two successive "Z" lines is considered as the functional unit of contraction and is called a sarcomere.

The striated appearance of myofibrils is due to the distribution pattern of two important proteins - Actin and Myosin. The light bands contain actin and the dark bands contain myosin.The light bands in a myofibril contain actin and is called I-band or Isotropic bandThe actin filaments are thinner as compared to the myosin filaments In a resting state, the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments.The "Z" line separates the sarcomeres in a myofibril.