Saving a page template is convenient way of saving a collection of configured elements, which you can then load onto multiple pages and edit as needed. If you are looking to create collections of elements which you would like to reference on multiple pages without changes, then an SPB Section would be more appropriate, see that section for more details.
To save a page template, simply create the page you wish to save, and then press the Save Template button in the Swift Page Builder header. See right:
Once saved, the page template will then be available in the Saved Items modal, under the Page Templates tab.