Amazon KDP's editor is fine if you only need a place to paste text and suffer quietly.
It is worse if you are actually publishing books.
Avant Garde exists because the final stretch of publishing is still full of clumsy tools, weak export, and interfaces built by people who do not seem to write long work themselves. The useful part is not glamorous. Keep chapters organized. Let the manuscript be heard, not just seen. Move sections around without tearing up the whole structure. Export cleanly. Reduce the amount of platform-specific nonsense between draft and publication. That is the whole case.
The listening layer matters more than most writing software admits. A chapter can look fine on a screen and then fall apart the second it is read aloud. Rhythm goes flat. Dialogue starts lying. A sentence you were feeling very proud of suddenly drags behind itself like wet laundry. Avant Garde bakes that humiliation into the workflow, which is a mercy.
The interface matters too, but not in the fake-spiritual way writing apps like to imply. Environment changes pace. Some sessions want cold focus. Some want warmth. Some want a little theatrical pressure. Neutrality is not automatically more honest than mood. The question is whether the software helps the manuscript move or merely asks to be admired for having opinions about typography.
That is where Avant Garde earns its keep. A practical machine for people who finish books and still need to turn them into something shippable.