Sounds like the OP is talking about an inline TOC (please correct me if I'm wrong). As theducks said, an inline TOC is not necessary in an epub, and IMHO it is not desirable. BUT, that said, many people are intending to convert the epub to mobi (aimed at Kindle -- why else would you bother?) and for that, an inline TOC is a big plus. Kindle doesn't make use of the toc.ncx file in any practical way. Nor, of course does Mobipocket reader.
If you convert epub -> mobi via calibre, you will get an inline TOC created from the epub toc.ncx file ( but you will lose some semantic information from the <guide> section).
If you convert epub -> mobi via kindlegen, no inline TOC will be created from the toc.ncx. Therefore if you haven't put one in manually, the mobi won't have one.
That, I think, is responsible for the interest in an inline TOC that is often expressed here in the sigil forum as well as the epub forum.
In my personal view (YMMV) this is mobi / kindle's problem, not that of the epub format. For a further discussion of the epub -> mobi conversion dilemma, there is an extensive thread here:
https://www.mobileread.com/forums/sho...d.php?t=102036
HTH