I did LaTeX -> pandoc -> print ready pdf & epub.

I really liked it because I have infinite layout control with LaTeX, and it’s a full blown programming language with if statements, operator overloading, functions, etc etc. makes it really easy to have one source document that does different things when outputting to different formats.

Not OP but probably to be able to tweak the layout without having a doctorate in LaTeX templates.

Presumably because the publisher takes the manuscript and does all the formatting and so on.