Digital content is a separate product when compared to the physical item. What you are paying for is many, many hours of developers assembling the various pages, handouts, characters, dynamic lighting (if you have a Pro or Plus subscription), etc. It's comparable to having the book a movie is based on: the content is mostly the same, but the format and presentation is different. Audio books and physical books might be an even better comparison. There is nothing stopping you from doing the uploading and setting up based on what you already own. Purchase of the Roll20 material is just a matter of convenience and time saving.