While Facebook has not issued a comment about this feature specifically, the company did say to TechCrunch: "Facebook internally tests products and features before they ship to the public so we can ensure the quality of the experience". In that case, there is still record that you did send a message and that you deleted it.
Facebook initially announced that it would be adding the "unsend" feature for everyone in April after it was revealed that messages CEO Mark Zuckerberg had sent to other users were removed from the users' inboxes while older messages that those users sent Zuckerberg were still there. One of the biggest questions surrounding this is sure to revolve around the amount of time users will have before being able to retract a message, and now, that seems to be a complete unknown. As it happens, someone in charge chose to unsend those messages, which is a clear misuse of power, especially as users themselves don't have access to such functionality. The feature will work exactly how the delete message work for WhatsApp application. So, basically, this feature follows the same principles of the "delete message for everyone" option in WhatsApp.
The unsend feature will be useful for people who mistakenly or intentionally sent an inappropriate message and want to erase evidence of their misdeeds. Deleting the message will remove the message from your end, but unsending will remove the message for both the parties. This feature is nothing new in the messaging/chatting Apps. With this feature, users could revoke a text if they wish.