The general reasoning given for this act by most of the people today is that “It brings Good Luck”.
However, scientifically speaking, in the ancient times, most of the currency used was made of copper unlike the stainless steel coins of today. Copper is a vital metal which is very useful to the human body. Water kept in copper containers is good for health.
In fact, Antimicrobial Copper and its alloys (e.g. brass and bronze) are registered with the US Environmental Protection Agency (EPA) as being able to kill greater than 99.9% of infection causing bacteria* within two hours of exposure
Storing water in copper and silver pots finds mention in ancient texts of Ayurveda for purification of water.
Throwing copper coins in the holy rivers was one of the ways our fore-fathers made to ensure us 99.9% infection free water, As rivers were the Main source of drinking water. Making it a custom ensured that all of us follow the practice. If coins remain in rivers for a long time then it becomes beneficial for those who drink it.
However coming to our generation, We are not using copper coins and The Pollution in our days is comparatively 1000’s of times more than those days. So, Please do understand that nothing is stupid belief in our Indian culture and traditions as per they mentioned.
More over, i think you would like to know that these days some countries are equipping metro trains with copper to make them anti-microbial. See, how other countries are researching and realizing the facts and how we are neglecting everything just because it is a part of our custom.
PS: Please do Search about with the keywords “Copper and it’s alloys properties” to know more.