Photoshop's Hard Mix blend mode seems nearly useless. It does have one 'killer application,' however.
The killer app for Photoshop's Hard Mix blend mode is creating halftones.
Here is a brief tutorial in how to create custom halftone effects in Photoshop using a gray scale photo, a grayscale pattern, and the hard mix blend mode.
I'm compiling a list of all Photoshop blend mode killer apps here

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