To bring out the eyes and add a little "pop".
To add drama. A crisp black and white treatment can make an image more dramatic.
Have a little fun. Here I wanted to give it a slightly vintage feel. Not too much. Just a little.
Or, a lot of processing is you want to go for a totally different look. I don't try to hide my editing. You look at this photo and you KNOW I messed with it. That's OK. I'm fine with that.
To bring in a little fill light. I needed Mina's face to come out of the shadows. I could have avoided this while shooting by throwing some more light up at her with a reflector or a flash. But, I didn't so I had to fix it in Photoshop. Besides I really liked the dramatic light on Jairo's face and I'm afraid if I had thrown more light in there I would have lost that. See how defined the angles of his face are because of the natural side lighting?
Sometimes you just want a little brightness and even out the colors more. The original was a little drab and I felt it had too much green to it. With a little tweaking I got something I liked a lot more with cooler tones.
To create the vision you saw in your mind. I knew what I wanted from this shot before I took it. I explained I wanted them with severe back-lighting because I wanted to make a silhouette of them. It took a few tries to get it dark enough in front, and I had to move them and myself a few times. But I finally got something I knew I could work with and with some Photoshop action I achieved the look I was chasing.