The page custom embed is used for when you want to share few recent post of your page or you want to share it without any javascript involved.
3 Examples
https://www.facebook.com/wpemf/
[embedfb https://www.facebook.com/wpemf/ social_plugin="false"]
https://www.facebook.com/BBCCulture/
[embedfb https://www.facebook.com/BBCCulture/ social_plugin="false" posts=2]
https://www.facebook.com/BBCCulture/
[embedfb https://www.facebook.com/BBCCulture/ social_plugin="false" posts=2 theme=elegant]
https://www.facebook.com/sydneyoperahouse/
[embedfb https://www.facebook.com/sydneyoperahouse/ social_plugin="false" theme=classic]