Why Do I Need Focusing Rails?
1x to 4x macro shots makes focusing very important. The focusing ring doesn’t help much. The whole camera needs to be moved to be properly focused. Parts of the flower only one millimeter away from the focal plane of focus can be very blurry. The camera needs to be moved very slowly to get the focus right.
The camera equipment becomes more like a microscope as the magnification increases. Consider a microscope which has solid geared focusing with the lens moving very slowly as it is focused. A macro focusing rail makes my camera work sort of like a microscope. The camera attaches to a focusing rail which is mounted on a tripod. A knob moves the camera and lens back and forth slowly from my subject. Focusing rails are expensive, but they make focusing much easier. It is easier to move the camera when focusing very close, and not use the focusing ring on the lens. Focusing rails are handier for 2X to 4X magnification photographs, but they can make 1x photos easier also.
Some focusing rails have a geared center column which can be easier to make very fine adjustments to focusing. Others simply slide forward freely. The important thing is being able to move the camera very slowly in small increments to focus correctly.