This post deliberately has no description in its front matter, which is what makes it
useful: it is the one entry in the index that exercises list.showSummary. With that
setting off, the listing shows the title and nothing else. With it on, the text you are
reading now is stripped of its markup, truncated, and used as the excerpt instead.
A description is still the better thing to write. It is the meta description, the card
summary and the feed entry all at once, so it is worth choosing the words rather than
letting the first two sentences of the body do the job by accident. The fallback exists
for the posts where nobody did, not as a replacement for doing it.