The "add one more" button from Drupal 8 default form only works when the client is connected to the server. Each click will get a response from the server and update its index and security data behind the scene. That's a lot of overhead in our use case, considering the operation stuff will click that button or using barcode scanning gun to trigger that button many times a minute. Also, they don't want to wait one ore two seconds, they want to bulk operate. Therefore, an implementation was added on the client side to match how D8 process form's meta data, therefore, no connection required until form submission.