You want a product (Let's say a bookshelf), so you search online, or you just pack your stuff and visit a mall or a shop.

 All product you find looks good and nice but still, you can't find what you want.  Maybe the bookshelf is too big, maybe your bookshelf is too soft, or they just don't have the color you like. 

You can wait for the product to appear, of course.  But if you want to take the matter into your own hands, 3D printing is a good start, and this is how I start my adventure.