The Preview Button initially displays the archetype/template in a depopulated form. But here you can type in some values and create an instance. Preview shows the datatypes of the node values indirectly, for example displaying units for Physical Quantities. Nodes that have a restricted number of decimal places, or choice constraints will also be displayed appropriately.
There are some other features of Preview that are useful:
This section only applies for the production version of the MO Template Editor. The free version has these features disabled.