How image upload works with Dropzone.js?

I guess if I have a form with action and class="dropzone, then after drop an image in the drop zone, fill upload to the specified image, right?

Why someone adds success event in init to Dropzone instead of add as action?

init: function() {
 landingDropzone = this
 this.on("processing", function(file) {
  this.options.url = file.uploadURL
 });
 this.on("success", function(file, responseText) {
  $.ajax({
  type: "POST",
  url: "/api/campaign/#(campaign.id)/client_image",

What if I do further manipulation of the image, i.e. I crop it with Cropper.js? Shall I use then success event?