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.
My work requires me to take certifications and keep them renewed.
What I appreciate from certifications is actually taking a well-made test. (Instead of a carelessly made normal school or university test.)
For well made certification tests, the question writing process is interesting and a whole specialization by itself. Each question is beta tested. And metrics are collected as to how a question is correlated or predicts that someone will pass the test. Tests should avoid mixing question pairs that hint the answer of another question. And writing multiple choice answers in which the answer is not possible to guess. Also writing questions so they are well written and easily understood. So that the question measures knowing or not knowing the topic, rather than affecting the test taker due to unclear wording.
I can see this being the case just for the fact that certs are usually paired a structured curriculum for learning a new skill. This takes a huge mental burden off and decreases time wasted in deadend exploration.