iPhone already has a strong lock screen, password, and Face ID experience. That protects the device before someone gets in. The everyday privacy problem is different: sometimes the phone is already unlocked, and you still need to hand it to another person.
That happens when a friend wants to see a photo, a colleague needs to scan a code, a family member borrows navigation, or a child uses the phone for a few minutes. In those moments, the lock screen is no longer the main boundary. The real boundary is around individual apps, photos, files, messages, and settings.
App-level locking gives you a second layer for these moments. Sensitive apps can be grouped, then locked before the phone leaves your hand. The other person can still use the parts you intended to share, without accidentally opening chats, work tools, or private media.
The value is not about distrusting every person around you. It is about making the phone match the situation. Your normal daily setup can stay convenient, while shared-use moments get stricter rules. That reduces awkward reminders and lets the boundary exist in the product instead of in conversation.
iPhone Locker is designed around this idea: keep the phone easy to use, but make sensitive entry points easier to protect when context changes.