Grayscale is a range of neutral tones from white through gray to black, without hue or saturation.
Converting a color image to grayscale removes the colorful hue distraction, allowing us to compare the lightness and darkness (value) of different colors without our color biases getting in the way.
(We offen assume yellow is automatically a light color and blue is a dark color without actually checking the value)
Two colors which look very different can look nearly identical in grayscale because they have similar values.
In the Lab, we use grayscale comparisons to evaluate the value of colored pencil swatches. This helps distinguish a pencil's actual lightness or darkness from the visual intensity of its hue.
