Often you want to import posts from another system, and use the unique key from the external records to ensure no duplicates are loaded in WP.
For example, WooCommerce has the "sku" field that is unique across all products. If you try and load a second product with the same SKU, it will update the original instead.
This CSV loader could benefit greatly from being able to do that. At the moment, if loading fails, you have no way to start that load again without duplicating what has already loaded, and that can leave a real mess in the database.