It promotes cellular respiration - In order for cells to survive, they need oxygen. Oxygen moves from areas of high concentration to low concentration, allowing the cell to respirate.
Nutrients pass through to cells - Cells need nutrients, such as glucose, to survive. Diffusion allows these nutrients to pass through to cells according to their needs.
It makes taking medication simpler - When people take medication in the form of a capsule, diffusion encourages the substance to move from the capsule and into the digestive system. Once there, diffusion allows it to move into the bloodstream. Without diffusion, people would have to rely on inconvenient methods of medication, like IVs. It also allows for innovative ways to medicate, like transdermal patches.
Diffusion in the form of active transport encourages cell signaling - In order for cells to function, they need signaling proteins. Not all proteins can move across the cell membrane via passive diffusion, but they can via active diffusion. When adenosine triphosphate donates a phosphate group to this practice, proteins can enter the cell.
It makes the nervous system work - Without diffusion in the form of the sodium-potassium pump, nerve cells would not innovate.