The media library allows you to store images, so that you can reference them directly in your CSV file (Bulk rotating and scheduling). This removes the step where you have to manually add attachments after uploading the CSV file.
If you are using the same image for different tweets, this can be a true time saver, as you only have to upload the image once.
After uploading an image it will appear in your media library. Underneath each image you will find a unique number (for example: "ID: 1234"). The 1234 in this example is the number that you need.
If you open your CSV file (Bulk rotating/scheduling), a column can be seen with the name of Media. Under this column, you can specify the number to indicate that this image should be attached to the corresponding tweet.
Tip: You can attach up to 4 images per tweet by separating the different numbers by a comma (,).