When an object is rendered it is actually converted to Triangles by the renderer and if you have four faces then the convertion is obvious (the faces will be split in half) and you know that it will convert following the flow that you have created, but if you have say five sides then it could be split in any number of ways and alter the flow of edge loops giving you nasty looking geomtery and deformation when you animate.
Why not use triangles you ask, well the answer is that you could but it is easier to model sticking ot quads in my opinon.What you want to avoid is ngons (faces with more than 4 sides) the odd triangle especially if hidden will not hurt you too much.