I‘m not a professional in Photoshop. But I can do as much as I need. Of course, a friend’s going to teach me it professionally.
Developers must be aware of Photoshop or other tools. I'm not saying very professionally. They need to learn as much as they can to get things done faster and better. 
For example, web and application developers only get a PSD file from the graphic designer. They have to convert it to code. Some places need to remove the color code of each part. Some places even need to take and save a piece of the photo or even the amount of shadow on each of the elements. So you need to be able to work with Photoshop as much as you need to.