> so their only purpose is cases where the content is actually found at multiple URLs
Most content can be retrieved using multiple URLs - for example this comment section is available at both https://news.ycombinator.com/item?id=26952557 and https://news.ycombinator.com/item?id=26952557&blah as well as infinite other variants. Even if you only actively link to one version, you can't be sure no one else will fuck it up so rel="canonical" is useful. og:url should just default to that though (no idea if it does).
Most content can be retrieved using multiple URLs - for example this comment section is available at both https://news.ycombinator.com/item?id=26952557 and https://news.ycombinator.com/item?id=26952557&blah as well as infinite other variants. Even if you only actively link to one version, you can't be sure no one else will fuck it up so rel="canonical" is useful. og:url should just default to that though (no idea if it does).