As far as the photographer is concerned, most of the Blending Modes in Photoshop are of little use and the menu of options could be whittled down to just five or six. And one of the Modes I'd keep is the Luminosity Blend Mode (LBM).

The LBM ignores all the colour values so when a Layer is set to LBM it blends with the Layers underneath using just the the brightness values of the LBM Layer. You can prove this yourself....open a colour picture in Photoshop, insert a new Layer underneath it and floodfill it with white paint. Now go back to your colour photo and blend it to the white Layer using LBM - it goes monochrome. Indeed some people use this method for B&W conversions, but I don't recommend it!

Anyway... click through these five Steps and see how use the LBM and the way its releases the "story" behind this old blue door and its rusty handle.
Guide to the Luminosity Blend Mode - Step 1Guide to the Luminosity Blend Mode - Step 2Guide to the Luminosity Blend Mode - Step 3Guide to the Luminosity Blend Mode - Step 4Guide to the Luminosity Blend Mode - Step 5