Assigning (targeting) an image’s highlight and shadow values is necessary because most output devices (usually printing presses) cannot print detail in the blackest shadow values (near level 0) or the whitest highlight values (near level 255). Specifying the minimum shadow level and maximum highlight level helps to bring the important shadow and highlight details within the gamut of the output device.

If you are printing an image on a desktop printer and your system is color-managed, you don’t need to set target values. The Photoshop color management system automatically makes adjustments to the image you see on the screen so that it prints properly on your profiled desktop printer.