Adding fingerprints to the default Android images would allow the testing of biometric tests using the Appium endpoints (eg. finger_touch) in tests with applications that use biometrics. The endpoint essentially allows the 'mocking' of a successful touch on the fingerprint reader of a real device.
Apps are increasingly incorporating biometric security features as part of login or other features inside their tests.
Appium supports the actual endpoints, but the emulator itself must have fingerprints enrolled, and settings selected for the use of a fingerprint. The latter is what is required to be implemented for this to work.