Membrane channels are important in allowing the rapid movement of molecules in and out of the cell, in a selective manner. By having a high selectivity, this allows the maintenance of concentration gradients for other substances (such as protons), but allowing the specific substance to move in and out unhindered.
Membrane channels facilitate the passive movement of molecules and ions down their concentration gradient, reducing the energy required for transport. They play an important role in accelerating diffusion - if all transport had to occur through the membrane, transport would be much slower and life would likely be less complex.