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One: For consistency, it also let's you know if the TOC is at the end of the book or not, and whether there is anything interesting before/after it.
Two: In this case they are different TOCs. It's a link in the off-line TOC to the in-line TOC. The in-line TOC can have additional information, like illustrations or short summaries, which are not in the off-line TOC, and there should be a way to reach the in-line TOC (if it exists) that does not involve paging forward/backward all the way.
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