When the phone is locked, the sensor doesn't really have a point.
When it's unlocked, it does - it is what tells the scanner to start reading when you set your finger on the home button to authenticate an iTunes purchase.
It doesn't have to, might not be... In the same way the software tells the keyboard to pop up for password entry, it could tell the scanner to come on when the touchid is requested within the App Store.
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At least two solid reasons have been given yet you keep ignoring them.
The primary one being that it's an additional security measure. If the software triggered the sensor, it would remove the 'human' element from the security measure.
I can use a pencil to press my home button and put my pinkie I'm, without ever touching any part of the metal ring and it still unlocks.