Why can I create more than one Site Studio template for the full view mode?

You can create multiple Site Studio templates for multiple reasons. 

The most common use case is to apply alternative styling on a different area of your template. Another use case is for providing different information based on the information available to the template. These are then available through the use of the Site Studio template selector field.

See the 'Managing your full content templates' for more information on managing multiple full content templates.