The plasma membrane is a bilipid layered membrane that allows lipid soluble substances to pass through. It is important that other substance pass through although they are not lipid soluble. In line with this, the membrane has specialized transport proteins in the membrane to facilitate the transfer of these non lipid soluble substances across the membrane.
It is also useful for the movement of such molecules and ions like glucose, important intracellular and extra cellular ions involved in the maintenance of electrochemical balance. This is reason proteins carry out facilitated diffusion. It is not a waste of energy in any way.
It is one of the ways by which the cell maintains the sanity of the cell. Cells even maximize energy by the use of facilitate diffusion when compared to active transport. Simple diffusion usually applies when the movements of molecules is along concentration gradient, just as in this is the case in facilitated diffusion. The use of proteins as carrier molecules is part of mechanism to maintain the homeostasis of the cell, to speed up the process of transportation and enable the cell survive in its habitat. When we compare this type of transport with active transport against concentration gradient, really minimal energy is used in the process.