When creating a new newsletter, defining a new page is required. If there is no newsletter page, one needs to be created manually.
However, when the newsletter is created, a new special page is created for that newsletter automatically. That page contains the title, description, a subscribe button... It also comes with a Talk page. For many newsletters, that might be all what they need.
Imagine that by default, the newsletter Main Page will be the new special page about to be created. If the publisher doesn't introduce any information, this will be it. If the publisher adds another page (as we do now), then the Main Page will be that one instead.