"Could someone assist me in grasping the function of these 'Page types'? Are they primarily designed to streamline the onboarding process for new developers? They seem to resemble a standard custom type, albeit with the inclusion of default uid and SEO tabs."
That's a great question! For now, the main difference is that page types have the default fields that you describe. As we move forward, page types will have more dedicated functionality, and you'll see other formats appear as well. For now, we recommend modeling your page documents with page types and other documents with custom types.
Sam