Traditional systems such as WordPress bundle the editor and the site. A headless setup keeps the editor and lets a fast, modern front end fetch the content, so the site can be static and the editing can be simple.
It suits businesses that want to edit content themselves without the maintenance burden and speed cost of a plugin heavy platform. It is more to set up and usually the right choice for a site expected to grow.
Sanity, Contentful and a headless WordPress are the usual choices; the front end can be rebuilt later without touching the content.