The goal of a Captivate <b>theme</b> is to keep a consistent design throughout your project. It can be 'applied' to any project, even <b><u>after</u></b> creation.
A <b>template</b> in Captivate has to be chosen <b><u>before</u></b><u></u> you create a project.
The term is often wrongly used: most Captivate 'templates' that you can find on the web, are just cptx-projects, not templates in the Captivate language.
When would I use a template in Captivate 9? For courses that have several modules, where you want to have some slides in common, maybe have custom navigation/control buttons that cannot be put on the master slide, but need to be timed for the rest of the project. I would rarely use it to have placeholder slides, unless some team members need to have that assistance.