Why are gradients important in diffusion and osmosis?
They allow for the movement of molecules without the active expenditure of energy
Molecules naturally want to go from an area of high concentration to an area of low concentration. This natural physical push to lower concentration is what drives molecules across membranes if they are small enough to fit through. The cell can use this natural inclination to move away from the crowding to move molecules around A good example is the H+/ proton gradient in mitochondria.
There are a lot of H+ outside the inner membrane of the mitochondria. Naturally, some want to come inside. ATP pumps lodged in the membrane have something like a turnstile/revolving door into the mitochondrial matrix (the inside). The H+ try to avoid the higher concentration and some push through the turnstile. It's the natural overcrowding that pushes H+ into the mitochondria, through the turnstile and then the mitochondrial inside (the matrix) where during other processes they are actively pumped out. The cell uses the turnstiles turning to help aid the creation of ATP