Thanks to Hugh for highlighting this insight from Kathy at Headrush: Your user's brain wants a conversation:

When you lecture or write using conversational language, your user's brain thinks it's in a REAL conversation!

In other words, if you use conversational language, the listener/reader's brain still thinks it has to hold up its end, so it pays more attention. It really is that simple, and that powerful (at least if you really want to help users pay attention and remember your message)...

If you're using formal language in a lecture, learning book (or marketing message, for that matter), you're worrying about how people perceive YOU. If you're thinking only about the USERS, on the other hand, you're probably using more conversational language.