Why is this on money?

I found out a couple of days ago that if you fold a 5,10, or 20 dollar bill, you can see he twin towers on 9/11. I wanted o see if it was true, and it is. I tried i on all three bills. One even has the pentagon burning. The towers all look different on each bill. I think on the 5 dollar bill it's almost gone. Why? Why do they put that on money?