Amazon recommend it and give these reasons
https://kindlegen.s3.amazonaws.com/A...Guidelines.pdf
Table of Contents Guidelines 3.3
Amazon strongly recommends the use of an HTML TOC for all books that would benefit from this navigation feature. This applies to most books, with the exception of fixed-layout children's books (see section 4) and fixed-layout graphic novels/manga/comics (see section 5).
3.3.1 TOC Guideline #1: Logical TOC Is Recommended
Amazon strongly recommends that all Kindle books include both logical and HTML TOCs. The logical TOC is very important for a good reading experience, because it allows a reader to navigate between chapters easily. Users expect to see an HTML TOC when paging through a book from the beginning, while the logical TOC is an additional way for users to navigate books. The inclusion of a logical TOC is especially important for books that are longer than 20 pages.