A title page is like a little “sub-document” within your screenplay document. It just isn’t visible all the time. You can switch to it to view it, edit it, etc. using Document > Title Page > Show Title Page.
Title pages can also have various parts of the text (title, author, etc.) bookmarked so that they can be found when adding placeholders to a header/footer, or when automatically generating the metadata for various output formats such as PDF.
The title page is always saved as part of your document. You can choose to include it or not when printing or creating a PDF.
(When editing the title page, Fade In will display a "Title Page" indicator at the top of the page. This is part of the output when printing/PDFing.)