I already bought the PDF format. Why should I get the ePUB or MOBI version?

The first reason is that it would be free (assuming we can find proof of your original purchase. See next question for details). The ePUB / MOBI versions have advantages over the PDF version when viewing the digital book on a reading device. The biggest advantage is that the text can flow to the exact size of your reading device so scrolling is not a problem. The ePUB / MOBI version also makes increasing the font size in your reader much easier.