Hi @whity,
The idea of the Thing Template is so that you can create a single AR experience for multiple IoT enabled Things that all have the same geometry.
Say you have 5 machines that are all the same but you want to view the live IoT data for each of them. Each of the machines will be represented by a Thing in ThingWorx that is built from a Thing Template. Within each Thing, you'll specify the ThingMark that is uses so the idea is that I can walk up to any machine, scan it's unique ThingMark and then I'll see the same experience for each machine, but showing the IoT data for the specific machine.
I didn't really understand this myself until I did the AR class from PTC that you can do for free in PTC University. You can take the class here: Fundamentals of AR Development with Vuforia Studio
Cheers,
Allan