They Have Stood the Test of Time
Ideas come and go, but only the best ideas are the ones that stand out. According to C.S. Lewis, as a writer, he would rather want a reader to read an old book than a new one as the new one is still under trial and the reader is amateur to judge it. His thoughts - "Often it cannot be fully understood without the knowledge of a good many other modern books. If you join at eleven o'clock a conversation which began at eight you will often not see the real bearing of what is said".
They Help Us Open Up More to Different Perspectives
Each generation has its own perspectives, preconceptions, and different ways of talking to the world. When we read old books, it helps us gain new perspectives and new ways to look at things, acting like a time machine which transports us to that period, and we could apply that newly gained 'Point of View' of that time in our current situation.