The worth of a thing is best known by the want

This is a generalisation of the more specific and well known: You never miss the water till the well runs dry. We normally take a lot of things for granted: parental affection, partner's faithfulness, job, bus service, etc. and its true importance is only felt when something goes wrong. It is then that we realise just how relevant and important the missing factor is to our lives and well-being.

This idea also touches upon the basic economic law of supply and demand. Scarcity pushes up prices and increasing supply lowers them. Some of the strange and unexpected behaviour of the stock and commodity markets can be explained by this simple law.

The word "want" has two meanings: "a lack of" or "a desire for", and the originator of the saying is using a play on words plus alliteration to emphasise the point.