The Classic Block allows you to add text content to your site using the traditional TinyMCE interface. The benefit of using the Classic Block instead of the Paragraph Block is that you can enter multiple paragraphs of text into the same block.

It’s also easier to add inline images and document links into the Classic Block as it provides you with an Add Media icon (Classic Block Add Media) that allows you to easily upload images and documents, in much the same way that you could in previous versions of WordPress, using the Classic Editor.

However since the Classic Editor has now been replaced by the Block Editor, it would be advisable to avoid using the Classic block.