Honestly, I enjoy learning but I don't like having nothing to show for it. Unstructured learning like going to a library and picking out a random book drives me nuts, I quickly feel overwhelmed. A cert at least gives me some goal to shoot for -- where I can prove some basic competency in the subject by virtue of having the cert.

I need something to "show" for my efforts. Project-based learning can work for me as well, but it takes extra work to figure it out for a given skill. I'm learning photography and I have a goal of doing 1 portrait session per week for a few months. That kind of thing helps me feel like I'm making measurable progress.