Currently ACF Blocks needs a PHP template to render the preview of the block in the Block Editor. However, for headless builders they are building their frontend templates not in PHP but in React or another framework. When building an ACF Block, the render template is in PHP. This is used on the frontend of the site as well in the backend as the preview when editing in the block editor. For some developers using frontend framework to render their blocks, having to recreate the same component in PHP so it can be rendered in the editor is a hassle. Allowing them to tell ACF to use the frontend component will make their lives easier and give the content editors the true preview of the block.
