Erik Kennedy - Learn UI Design

Let's break it down:
UX Designers. You can present designs as beautiful mockups that your coworkers will rally around. You can work with interfaces from concept to pixel-perfection. Your portfolio stands light-years ahead of your peers.
Developers. You can cover for design on your team. You can spiff up your side-project's interface, instead of finding/hiring a designer. You can translate designs to code with minimal hassle, because you understand the aesthetic underpinnings.
PMs. You can create amazing mockups for pitching new features and flows. You can work alongside your designers, and provide spot-on feedback for excellent product.
Entrepreneurs. Like it or not, a good presentation for your ideas matters. Your marketing site, your app - even your pitch deck. Eventually, you'll be in the big leagues and can hire out - but even then, a solid foundation in design will help you communicate and lead.
Print Designers. More and more design work shifts to digital every day. You've got to learn the ropes, figure out this whole "responsive" business, and practically re-build your portfolio to keep working in the field you love.
For me, I had left my job to become a freelance UX designer, and my portfolio looked about as good as a Pentagon Powerpoint. Not to mention every one of my clients was asking: "hey, now that you made these wireframes, can you do the actual designs?" It was clear I could offer more to my clients if my end product wasn't sketched out boxes and arrows.