I spent a lot of time in Illustrator and Photoshop. I first learned when I was a technical writer to improve the look of our docs, but then went on to become a graphic designer who also wrote things sometimes.
I wanted to include these skills in a front-end development portfolio primarily to show that I have some knowledge of design best practices and can employ them when needed. This makes me less dependent on a designer because I like to think I can make things look decent without a great deal of oversight from someone specifically invested in the aesthetics division. Secondarily, but perhaps as important depending on the organization, I include design knowhow to show that am capable of mocking things up, working on images (resizing and compression), and creating flowcharts for internal purposes.