If your image is smaller than 600 pixels wide, Facebook will shrink it into a tiny thumbnail on the left side of the text. This drastically reduces your Click-Through Rate (CTR).
By using the full 1200×630 size, you force platforms to generate the Large Image Card, which looks like a full-width banner in the news feed. This makes your link look authoritative and inviting.
Conclusion
This is arguably the most important “hidden” image size in digital marketing. You don’t post it directly to your feed, but it represents your brand every time someone shares your URL.