Why is it important to recognize old knowledge?

Why is it important to recognize old knowledge?
This from Dr William Osler

1. But to consult old books gives us more than a literary or an ethical satisfaction. The study of old literature teaches us many things:
   
2.  They knew things.
   
3. If they had been remembered much labor would have been saved.
   
4. They would teach us modesty, proving that much that is called "new" is very old indeed.
   
5.  They would give us respect for our ancestors and predecessors.
   
6.  It makes us respect our race of all ages, and appreciate the slow but steady progress to which 1000 men contributed almost everything, and we only a mite.
   
7. Let me give you a few fragments of ancient medicine. You will find much in it that you have considered modern. You will have to excuse, however, the fragmentary character of all my tales. My first reminiscence concerns hydrotherapy.
   
8. The Cleveland Medical Journal, Volume 5
   
9. Dr William Osler FRS FRCP