It allows you to explore topics that are well understood, in a way that fits your own understanding and pace. It's like somebody writing a great mini-tutorial on topics you're interested in, in a pace and abstraction that suits you.
Examples for me are concepts of mathematics or computer science that I would like to freshen up on. Things you could also ask a colleague over lunch, or find eventually via searching Google/Youtube/Wikipedia etc. Just much faster and more convenient.
Often I have a specific question like how does X relate to Y. And usually the answer given is total nonsense.