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The entries are to allow them to build the TOC and do navigation rereading the book. But, I think it is overkill now. The book is being opened to be read, so why not process the TOC at that time.
For kepubs, there is a some extra data. They store the word count for each chapter, but this is calculated the first time the book is opened. These also have the time estimates, but again, these are calculated while reading.
The only other use of the TOC rows for epubs, is recording the current position. The main row, has which of the chapters you are up to. The chapter row has where in the chapter. For kepubs, this is all stored on the main row.
I suspect these are carried over from older devices and firmware. It might have been better to do it this way with slower devices. Or they might have had an option to display the TOC without opening the book. Again, for a slower device, that might have been useful. But, apart from "don't fix what ain't broke", I don't think this is a useful mechanism now.
And while I wrote the above, a picture came into my mind: a group of Kobo developers reading my post and shaking their heads and saying "You have no idea what you are talking about".
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