In reply to ts.

This may have a different answer for different folks, but the biggest benefit to me is the standardization across the platform. When WordPress, plugins, themes, and users can all universally communicate via the block and global styles systems, they can work together in ways that they never have been able to do before. This can surface itself in many ways. Sometimes, it means fewer scripts and styles necessary on the front end. It can mean a plugin's front-end output matching the theme's design. Maybe it is a more accurate WYSIWYG post editor. Or, countless other improvements, some of which we probably haven't even thought about yet.

Using a block theme doesn't mean you need to customize your site's design no more than classic themes require it. However, users will still benefit from the standardization under the hood.