Explain why to write an explainer

In https://github.com/w3ctag/tag.w3.org/pull/39#pullrequestreview-1541500846, @chrishtr suggested

> I think it would be very useful to expand on this with a paragraph or two on "why write an explainer?" They are indeed very useful, but a lot of people don't understand why, and it'd be very useful to have a compelling explanation, ideally with a worked example.