Templates are typically built using smaller components. Structural containers hold modules, images, text, etc. Most templates have their own layouts.
The layout builder allows you to build structural layout presets. When building a template, you can use the layout presets to quickly apply a desired structure with all of its containers. Simply drag/drop the prebuilt layout into the template and the entire structure is created.
NOTE: The layout presets are typically used right at the beginning of a template creation. They are intended to make template design faster. Layout Presets are only used for quick drag/drop design in templates. The layout presets can't contain content or modules outside of templates.
To create a Layout Preset:
Applying a Layout Preset