Repeating one digit of a 4-digit PIN can mean the difference between a phone that's safer and a phone that can be unlocked at a glance.
Photo via Ars Technica.
With a passcode, it is often very obvious which numbers you have tapped because of the finger smudges, but you don't know the order those numbers were tapped. A PIN limited to 4 unique numbers only has 24 possible combinations, but a PIN with three uniques and one repeated number has 36—which means those fingerprint smudges of yours won't betray you quite so easily. Just be sure the PIN you do use isn't 1998.