Unlike other currencies precious metals have continued to rise in value over the last 2 decades (and yes, at present silver has dropped nearly 40% in 12 months, greatly due to being over sold so the rebound is potentially huge). In contrast to stocks and bonds one can buy on the stock market Copper, Silver & Gold are not regulated by CEO/managerial decision-makers, and the fact that they are ever rare is what makes them precious!
Where as bonds and stocks are for all intense and purposes just pieces of paper, saying that you have invested a particular sum of money in a company. Precious metals on the other hand have been valued by civilisations for thousands of years, they are in our psyche, they're wonderful to hold and are beautiful to the eye, and this is what puts a value on them above and beyond that of money.
You only have to hold a 1oz Silver coin in one hand and a £20 note in the other to appreciate why precious metal has real value unlike printed money!