_wp_add_additional_image_sizes is private and should not be used in themes or plugins directly.
Add additional default image sub-sizes.These sizes are meant to enhance the way WordPress displays images on the front-end on larger, high-density devices. They make it possible to generate more suitable
sizesattributes when the users upload large images. The sizes can be changed or removed by themes and plugins but that is not recommended. The size "names" reflect the image dimensions, so changing the sizes would be quite misleading.