Styles API Module allows other modules or themes to register theme templates on a YAML file (replacing hook_theme()) and provide a repository for all themes registered.
This will give more order to theme definitions, taking them away form code.
But moreover in this way themes and modules could gather information of registered themes implementation.
For example a module could add select field to forms, providing a UI to easily assign predefined templates for elements, blocks or regions.
Among standard hook_theme() settings, the definition will add Admin UI elements like labels, icons, categories, descriptions.
While Block Styles Module using the Styles API Module allows to set different styles for block wrapper theme.
This module affects only block.html.twig, and not the way content is displayed.