An improved model of the structure of cell membrane was proposed by Singer and Nicolson (1972) is widely accepted as a fluid mosaic model.
According to this, the quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer.
This ability to move within the membrane is measured as its fluidity.
The fluid nature of the membrane is also important from the point of view of functions like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division, etc.