This feature is used to prevent identity theft, stop unwanted attention from strangers, and protect against cyberbullying. Common "Viewer" Tools & Methods
Asking for your own Facebook credentials to "authenticate" you, which then allows them to steal your account. facebook locked profile viewer online
In short: A locked profile is Facebook’s way of saying, "This person does not want strangers or casual acquaintances browsing their personal life." This feature is used to prevent identity theft,