The reason is; according to the Facebook help center, when you contact the developer of an app or game, you may need to give them your user ID to help them better address your question or concern.
Another vital thing you’re expected to note here is that some apps and games developers may direct you to their own help information. See: Here’s What to do if your Facebook ad isn’t approved
It’ll also interest you to know that apps and games are managed by developers outside of Facebook. So if you need help with an app or game, you’ll need to contact the developer.
How do I contact the developer of an app or game?
To contact the developer of an app or game, do this:
1. You’ll need to open the app or game.
2. Finally, click Report/Contact in the bottom-right corner.
See: How Do You Manage your payment methods for Facebook Ads?
How to Find your User ID for your Apps & Games on Facebook
To locate your user ID for your apps and games, do the following;
1. Click the V-like icon in the top right of Facebook
2. Next, click Settings from the drop-down menu
3. Click Apps and Websites in the left menu.
4. Hover over an app or game and click View and edit.
5. Finally, navigate down to Get Help From App Developers. Your user ID is in the paragraph below.
See: Where do I see all the people I’ve blocked on Facebook?
How to Find your User ID for your Apps & Games on Facebook