Copper has anti-bacterial material property which acts as a purifier as soon as it touches water. Copper has the effect of killing germs in water. In ancient times the coins used to be made of copper and they would gradually dissolve in water settling all the germs in the bottom and leaving the fresh cleansed water on top.
The US Environmental Protection Agency survey says that when coper is exposed to water for more than 2 hours it is able to kill more than 99.9% of the infectious causing bacteria. People in the past used copper coins to purify the water as rivers were the only source of drinking water.
The actual reason for throwing copper coins was to purify the water of the rivers. However people began to believe this as a means to ask for wishes. Since then people have been mirroring this act and continuing it for ages to ask for wishes from God.
People followed the law of ‘karma’. It is believed that ‘what you sow is what you reap or what goes around comes around. Thus when people throw coins they get back more coins (wealth). It was believed that they would get more wealth by throwing coins in rivers.
The act of throwing coins was a way of feeding Lakshmi or Goddess of wealth. This was like her replenishment.
In modern times the coins are no longer made of copper and people still are continuing this tradition for so many years. In a way it is like wasting your wealth. However this is a good source of income for the unprivileged people who collect these coins at the end of the day near river banks.