The justification of biometrics is usually to make it harder for fraudsters to pretend they're someone else.
However, one advantage of something external to your body is that if it ever does get compromised it is a lot easier to deprecate that thing and replace it with a new one.
In practical terms, biometrics are almost always introduced as a convenient way to authenticate, certainly not as a MORE secure method of authentication. Just look at how protestors have disabled it to avoid giving cops an easy way of accessing their phone without their consent.
