Most are clickbait designed to show you ads without delivering any results. Privacy Policy:

This feature is especially common among women, minors, and public figures who want to avoid harassment, stalking, or identity theft. Locking a profile is a deliberate act of privacy protection – and Facebook enforces it strictly.

They force you to click ads or complete endless surveys to "unlock" the image, generating ad revenue for the scammers while leaving you with nothing.

Right-click the profile picture and select .