Entering a passcode basically unwraps an encryption key that's used to secure files on your device. On pre-Touch ID devices, locking the device throws away the unwrapped key, requiring you to enter your passcode again to unlock the phone.
On Touch ID devices, the unwrapped key is handed off to the Touch ID subsystem when the device is locked. So if you restart your phone, the Touch ID subsystem has no key to provide to unlock the phone. You need to enter the passcode at least once so it has the key to unlock the phone.
The Touch ID subsystem also deletes the key after 5 unsuccessful fingerprint reads, or after 48 hours of the phone being locked. It also deletes the key if Lost Mode is activated via Find My iPhone.
This is all for security, in case your phone is stolen and they try to use a copy of your print to unlock it, or to force your finger onto the sensor to unlock your phone against your will.
Touch ID is not more secure than a complex alphanumeric passcode (although it is more secure than a 4 digit PIN), but serves as a relatively secure mechanism to prove that you're the person that knows the passcode. It's not a complete replacement, just a convenient alternative.