If I ask you how to make some information from a product page display in a pop-up you will most probably say that it is an easy task to do. Just pass the product id via ajax-request and then load the product object in the controller and thus you can generate any information.
Yes, this is really easy to implement. But what if I want something more? For example, if I want to display the content exactly in the same way it is presented on the product page. Because there can be several modules enabled on the store which influence the product page content, so I would like the pop-up to take all of them into account.