I use them to create a sense of foreboding, like its destiny that drives them and all is already plotted out.
Furthermore the heroe's get an indication how far they are away from the final cataclystic event that concludes the campagne. It helpes to give heroes a certain feeling over the time line of the campagne.
Last but not least, it is always fun to descover hidden writings with obscure meaning that predict the future. (Nostradamus wrote big bestsellers in this concept :)) )