Why there's a PIN
Your cloud credentials live in this device's secure storage. If your device has no lock screen, the app uses a PIN as the lock protecting that stored token, so a found or borrowed phone can't open your accounts.
Setting or changing it
Set a PIN of 4 to 8 digits during setup, or change it any time from App settings → Change PIN. You enter it twice to confirm. The PIN is stored only as a salted hash — never in plain text, and never sent anywhere.