Gold and Silver are the only assets that, once purchased, is completely in your control and power. Once you take possession of gold and silver, you no longer have a middleman attempting to, or able to take the value of your asset from you.

Stocks, bonds, and even cash are really under the control of Central banks and financial institutions that can and do play with the value and worth of these assets at will. History shows multiple examples of market manipulations and illegal acts that have led to the destruction of wealth overnight.  Once in your possession, they are out of the system and cannot be taken by manipulation of the paper networks or bank actions. Gold and silver cannot be watered down in value, like money, by a Central Bank that can print money without limits and are immune from political demands to inflate the money and thus secretly steal your wealth. 

Gold and silver are your shields of protection in troubled times when politicians look to take stored wealth through taxation and inflation.