Convex mirror is used as rear view mirror in vehicles. Because the image is smaller, more image can fit onto the mirror, so a convex mirror provides for a larger field of view than a plane mirror. This is why they are useful.
The Indus Valley Civilization which is also known as Bronze age civilization started around 3300 BC and flourished in between 2600 - 1900 BC and started to decline in 1800 BC.