Facebook comments widget – App ID

You’ll be able to moderate comments. Here’s the relevant quote from the doc‘s Moderation Tools section:

Doing this enables a moderator interface on Facebook where comments from all plugins administered by your app id can be easily moderated together.

I personally never used the moderation tool, but it is available if you need it, which is nearly always the case if you are working with a corporate client who would want to have control over all site content, including comments.