A password alone is not enough for a phone-number account
Messaging apps tied to a phone number rely on SMS verification by default, and SMS codes can be intercepted through SIM-swapping or carrier-level attacks. A PIN that only you know closes that gap.
How this differs from general two-factor authentication
This is a messaging-app-specific setting, distinct from the general concept of two-factor authentication used across many kinds of accounts. The underlying idea is the same, but here it applies specifically to protecting the phone number tied to your messaging app.
Frequently Asked Questions
What happens if I forget my PIN?
If you added a recovery email, WhatsApp sends a reset link. Without one, you can still re-register your number, but you'll need to wait roughly a week before the PIN requirement is lifted.
Will I be asked for the PIN every time I open the app?
No -- it's requested periodically (roughly every few weeks) and whenever you register your number on a new device, not on every normal login.