A Metabox Block is a CJT block that is assigned to one single WordPress page or post. This type of block cannot be assigned to multiple items (i.e. pages/posts) at the same time. The main visual difference between a Metabox Block to a standard Global Block is a Metabox Block does not have the Assignment Panel.
These blocks are created at the page/post level (i.e. when editing a page or post). They can be useful when working with the post content and having the code in the same place is required. It can also provide better code management, if say your code organisation requires each block to be assigned to a specific post, and you want to leave the Global Blocks for global cases. Metabox Blocks provides another option to help you manage your development in a structured way.
Posts/pages do not have Metabox Blocks created by default. Users have to create a Metabox Block for each post that requires it. This is intentional as it allows for better performance for posts/pages that do not need them.