What is this "sent by ManyChat" message doing here?

Here is an example:

This is an automatic prompt that Facebook puts into each sent message from your Facebook Business Page to help admins understand who is sending each message.

Anybody who has admin access/permissions on your FB page can send Messages.

Your users and subscribers will NOT see this message, it's only a silent indicator on the ADMIN side of things.

Also, this is exclusive to the FB page manager, and will not display at all in the ManyChat live-chat feature right now.

This is just a formality FB has to help page owners understand who is sending what from their account.

Hope that helps explain things!