To elaborate on what Hawkeye said, here are some ways it's different from simply using layer blending modes:
1. It provides two other blending modes, "add", and "subtract"
2. It allows one to blend images from different files, not just different layers.
3. It allows one to directly blend images that are in different color modes, eg, an RGB image with a grayscale image.
4. It provides a *very* convenient / quick way to specify the source and destination channels, even composite channels. Using the "calculations" and other commands one often has to do some extra intermediate steps (eg, copy the contents of a channel to its own layer or to a new user defined channel, etc.).
5. It's the tool of choice for frequency separation skin smoothing.
Cheers,
Tom M