Well, perhaps out of habit, tradition, there would even be some who would attribute it to some kind of superstition... but the fact is that it has a scientific explanation.
During the 19th century, in the so-called colonization of the Far West, the caravans of settlers had to travel long distances through vast plains and virgin territories for the white man. The ignorance of the orography and especially of places where to supply water gave special importance to the transfer of large quantities of water and, above all, its conservation. The conservation method, although it seems strange, was to add a silver coin to the water and milk. According to what Herodotus tells us, this method of water conservation was already used by Cyrus II the Great of Persia (6th century BC), that in his multiple conquest expeditions he always carried water in large silver vessels.
And the scientific explanation :Silver is a powerful antimicrobial agent that prevents the growth of microorganisms that spoil foods and beverages. In addition, it is tasteless, odourless, non-toxic and is used to treat more than 600 viral and bacterial diseases (parasites, herpes, candida...). Surgeons even used silver thread for its bactericidal properties.