Diffusion is one of the fundamental processes by which material moves.
It is thus important in biology and medicine, chemistry and geology, engineering and physics, and in just about every aspect of our lives. Diffusion is a consequence of the constant thermal motion of atoms, molecules, and particles, and results in material moving from areas of high to low concentration. Thus the end result of diffusion would be a constant concentration, throughout space, of each of the components in the environment. Of course there are also processes that generate inhomogeneity, even while diffusion is smoothing things out, and the world we live in is the sum of both. In most systems the energy is just equal to a constant times the temperature.