When creating Event Categories and Event Sub Categories, you can select a default Event Template for each.
When you later book an event, you can assign an Event Category and Subcategory to it. If the Category or Subcategory has a default Event Template, the details of that template will be added to the event automatically, without additional typing and selection on your part. This saves you time and effort, and ensures that important details aren't overlooked.
If you want the Event Categories and Subcategories to work that way, create your Event Templates here and return to Event Categories and Event Sub Categories in Setup to assign the templates to the relevant categories.