Bright parts of an image will draw the viewers attention, so you want to make sure nothing distracting is catching the eye.
A common way to view lightness is to set it to black and white – but often it is very hard to judge how bright an area is in relation to other parts of the image.
Adding this gradient map will create a “heat map” that maps out lightness in your image. As you can see in the dialog below, colours go from cold to warm, with black and white on the far ends.
The red parts in the image above all have the same lightness. This would be very hard to spot in black and white.
I also use this method after separating out saturation to judge it the same way I do with lightness.
To create the gradient map, simply install my Photoshop action:
Download it here