A printed page number is a physical coordinate. On a phone, tablet and wide monitor, the same paragraph occupies different space. Pretending otherwise makes a digital book harder to use.
A book needs two maps
The table of contents preserves the author's intended journey. The subject index serves readers who arrive with a person, place, organisation or idea already in mind. Both should lead to precise, durable sections.
Every important passage can be found again
Stable section anchors allow references, messages and AI-generated answers to point at the relevant passage, not merely the book cover. Revisions may change the words around an anchor, but should not casually destroy the address.
The index is not an appendix to an online book. It is part of the interface.