To edit it. EPUB ní XHTML tí a lè pàṣẹ̀ padà nínú zíp - àkọ́lé tí a fi pàṣẹ́ padà padà sí ìwọ̀n ojú-ìwé wònyí, eyi ní idì tí àwọn awáròyìn e-ìwé fẹ́ rẹ̀, which is optimised for reading rather than revising; a document format is what you need to make changes, add comments, run a word count or hand the manuscript to an editor.
Illustrations, diagrams and the cover are extracted and embedded in the RTF at their original resolution, positioned in the text where they occur. Formats that link rather than embed get the images written alongside.