This likely stems from my complete lack of familiarity with encryption technology and IT security in general, however it isn't clear to me how biometric authentication (such as Apple's TouchID) makes the data it protects more secure than a simple password.
It's clear to me that, individually, biometric authentication is more secure than a memorable passcode. A fingerprint, face or voice can't really be "guessed", for example, in the same way a password can, and is characterized by something like thousands or millions of datapoints. However, biometric authentication systems such as TouchID often only complement a simple passcode. If, for whatever reason, I'm unable to unlock my iPhone with my face or thumb, I can still unlock it with a 4-digit passcode.
Since e.g. TouchID only adds another way to unlock e.g. an iPhone, isn't the protected data in principle easier to "hack" (and, in practice, something like just as difficult)? There are now two "entryways".
The main reason for Apple to introduce TouchID was to make people use more complex passwords. For the sake of quick and easy access to their phones, people often used very simple passwords or no passwords at all, because they found it impractical to type in long passwords.
With TouchID, it became possible to use long and thus more secure passwords, while still being able to quickly and easily access the phone with just a fingers touch.
So, while TouchID does not add security by itself, its practical use allows to improve the security of the existing protection method.