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As to why there are no parallels between aria roles and epub:types I have no idea. I think aria roles spec came after the first Epub Semantic Structure Vocabulary spec.
Without that parallelism things do not make much sense at times. The rules and examples are arcane at best.
But think of a Screen Reader that sees a section with a role. If the section has a aria-label, it will announce that label and not say a generic Section role. If it has an aria-labelledby attribute that points to a heading tag, it will read that instead., etc.
The key is not to make the screen reader repeat ... saying things like "Chapter" immediately followed by "Chapter 1 A New Beginning". The latter is the one most listeners would prefer.
The sections are quickly indexed by the screen reader to help them quickly navigate among sections within a single file.
All of these arcane rules on usage and placement are trying to ease access to different sections while at the same time making Screen reading easier for the listener.
It makes a bit of sense then. But of course, loading your epub into an e-reader that supports specialized accessibility software and listening and trying to navigate is the only sure way to know you have improved accessibility.
Hope something here helps.
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