Diffusion is essential for cells to bring nutrients and releasing wastes WITHOUT spending energy. This is accomplished by the "urge" of a system to achieve equilibrium, which means that materials tend to flow from high concentration to low concentration area. For example, if a cell lacks amino acids, and therefore has a low concentration of it inside the cell compared to the outside, then by diffusion, the amino acids outside will flow in through the membrane until the concentrations of amino acids outside and inside the cell are the same. All these are done without spending energy from the cell. Note, however, that simple diffusion cannot transport larger particles or those insoluble in lipids; these materials have to be transported with facilitated diffusion, where a carrier on the membrane of the cell will help bring in the materials.
Diffusion is when particles in a more concentrated area move to a less concentrated area, through a semi- permeable membrane. Without diffusion, the cells in your body will not be able to obtain the resources they need to survive, like water and proteins.