An alternative to asking a question, the "how to" headline is another way to open a curiosity gap. It also suggests that your content is going to be practical, so it's important to meet that expectation if you don't want your reader to bounce as soon as they realize they've been duped.. This can also be said of “what,” “how,” and “why.”
In a survey by Conductor "how to" headlines were voted the third most clickable, and almost made second place.
Why are they so popular? It all comes down to clarity – people are drowning in information online. They want to learn new things and a direct "how to" headline allows them to instantly see whether that's something they want to learn or not.