If a linked webpage is missing description or image metadata, Discourse will normally leave it as a URL and not onebox it. This plugin changes that behavior to allow for title-only oneboxes.
The description requirement was actually removed at one point (way back when it was a standalone gem) but then it was reverted a few months later, so I assume there was some reasoning not to do it.
I have only seen a handful of URLs in the wild that match this scenario, so I imagine there are some edge cases where it doesn’t look right.