Normally, an ebook doesn't have images, or if it does there is no way to specify where they appear (so just including them in the html is fine)
In a professionally produced print book, images are positioned precisely with margins, alignment and text flow. The Microsoft Rich Text control (which is what yWriter uses) supports none of this. Therefore, there's no way to export to DocX or RTF with proper layout.
However, LaTeX does support this, and that's why LaTeX is preferred for outputting to PDF for printing.
yWriter was originally designed just to write the text of a novel. Then, in 2010 (10 years after yWriter was designed), ebooks started to gain traction so I added export to ebook. Export to print was never going to be a thing, but then print on demand came along, so I added the LaTeX export for precise formatting, inclusion of images, text flow, headers, footers, footnotes, etc. (All via codes and the latex header file, not visual wysiwyg).
Not only that, but using global and project variables means you can reuse elements throughout your novel and change them all in one place, instead of (e.g.) resizing dozens or hundreds of photos in a document by 5% one by one.