Diffusion is important to the cell because it is the process by which useful molecules enter the cell and waste products removed from the cell.
Without diffusion, cells and tissues of the body can not get important nutrients and gases for survival and would die.
An example is the eye. The eyes lack a great number of blood vessels (which carry oxygen that diffuses into cells) and must have an extra supply of oxygen. The atmosphere provides the extra needed oxygen which is taken up into the cells of the eye by diffusing through the cornea and thus the eye is kept functioning. If this does not happen , the eye would dry out.