I use a MacBeth color panel on the first frame of my studio work. That makes it a *known standard* for color, for product photography, I shoot a MacBeth as well as a grey step tablet. This way, since I know the colors of these items, I can calibrate my RA-4 process to produce final prints correctly.
Because a grey shirt with red pants can look like different colors of grey and red to one person, then different to another, the human eye cannot be considered accurate. The human eye adjusts to any given tone, a dirty white, if you stare at it long enough, will become pure white (at least that is what it seems like). So, you have to have a known standard to calibrate to. That's what the step tablet does. The MacBeth allows you to calibrate to a known cyan, magenta, yellow, grey, white, red, blue, green, etc.
Therefore, you take out almost all of the variance at the printing stage.