Rationale for chapter template change from 3.x and beyond

Hi

I was browsing the mail archive to look for the reasons that leaded to
the change of the templates of the Guides. I could not find the
motivation (*).

Except for visual change of the pages, I can't imagine the reason of
changing the names of the styles. Can someone point me to the
discussion, or summarize here?

Thanks
Olivier

(*) https://goo.gl/5Wkibh