The correction of spherical aberration depends on the distance at which you're focusing. Lenses without floating elements are usually optimised somewhere around hyperfocus or at portrait distances depending on their expected usage whereas a good macro lens will have a floating (moving) element that allows it to be corrected for nearly any distance. For 35mm systems it happens automatically (there are different groups of elements in the lens on separate helicals, you can often see them moving independently when you wind the focus ring) but for Mamiya M-LA lenses, you need to manually set the floating element with an extra ring on the lens because the lens doesn't know how far out on the bellows/tubes it is.
Originally Posted by StoneNYC
If you stick a normal lens on a long bellows, you will get a high magnification but also a very soft image with a curved focal plane and some crazy aberrations. If you use a proper macro lens, it will be sharp throughout the image with a flat focal plane, so you can actually reproduce a flat image accurately. You can focus near the corner of the frame and actually achieve some sharpness.