The above clause comes from Article 1, Section 10 of our Constitution, in which, quite obviously it makes it clear that the representatives of the various States, believed strongly that it was necessary and proper to restrict all of the individual States to a sound monetary policy. The main reason that this was so, was because during the revolutionary period, continental currency was notorious for not being worth its value as printed on that currency, and a country without stability or control of its currency is often going to be a country that will find it hard to conduct business locally, as well as across State lines, net alone internationally.
Today, however, the currency of the realm in the United States, known as Federal Reserve notes, are not back by specie, such as gold or silver, nor are those dollars backed by anything of real value or substance, except for such inchoate concepts as being backed by the full faith and credit of the United States government. Not too surprisingly, since the United States completely left the gold standard in 1971 the inflation or the worth of our currency has plummeted drastically, in which it takes in the present day, as calculated by in2013dollars.com, around $6.37 to equal what $1.00 was in 1971, or the worth of $1 today, is the equivalency of 15 cents, back in 1971.
The whole point of attaching specie to paper money is to help to not only stabilize paper currency, but also to prevent the national government from forgoing their duty to keep their fiscal house in order. The bottom line is that paper currencies that are backed by essentially nothing, eventually reach the point where the confidence in the value of that paper currency, disintegrates to such a degree, that governments, have to thereby replace or revalue that currency to reflect its true worth on a national, as well as on an international scale.
Those that wrote the U.S. Constitution and ratified such, were no fools, and understood well that sound money and fiscal responsibility were part and parcel of what nations' needed in order to establish and to maintain a firm monetary foundation. This so signifies that America, clearly is on the pathway to some sort of monetary upheaval of truly epic proportions, for when any currency is stripped of its stabilizing value given through specie or something of that equivalency, and replaced instead with essentially nothing of lasting substance, than all sorts of trouble will thereby ensue.
This does not mean that all is lost, but what it does mean is that there will most definitely be losers when the current monetary system is replaced with something that will be based upon restraining prudently the monetary printing press, which currently seems to operate in America, 24/7. No country can simply print fiat money as its way to keep its fiscal house in order, without at some future point, having to face the true consequences of their actions. That reckoning will surely come, and those monetary houses built upon foundations of sand, will not well stand the towering terrible storms that will inevitably come their way.