Why does S7 prompts for PIN/Pattern/Password when unlocking device using Finger print?

To use the Fingerprint function on devices based on Android OS 6.0 (Marshmallow), you need to set up PIN, Pattern or Password that must be set first. This is Google’s new concept to reinforce the device security and it’s common concept for devices based on Android OS 6.0.

Accordingly, you can unlock your device with PIN, Pattern or Password which you have set, even if you set your lock screen as Fingerprint.