I just got the new MacBook Pro with the touch bar, and I do not know what the TouchID button does. It is clickable, but as far as I can tell it does not do anything when clicked. However, this is unlikely because I don't think that Apple will add an inoperative button to their computers.
You can do multiple things with TouchID on a MacBook, it's essentially the same thing as on the iPhone.
With TouchID you can:
- Turn on, restart, sleep and shutdown the system.
- Unlock and log in when waking your Mac from sleep. You must type in your password after start up, restarting, or logging out of your account
- Gain access to password protected sections, such as in System Preferences.
- Make purchases in the iTunes Store, App Store or iBooks store.
- Making purchases with Apple Pay in Safari
Although 2017 MacBooks start when you open the lid, the finger print button also acts as a power button, like all other laptops. You can hold it down to force shutdown and restart the OS.
I'm sure there is also more things that I'm unaware of.
That button is very useful. As another way to login to this machine you need to enrol in TouchID within iCloud/System Preferences. You will then be able to login to your MacBook using only your fingerprint.