Gold, silver and copper are considered pure elements. It is believed that these metals can never be impure. Even yantra used in puja's are made of these metals.
Gold and silver are costlier as compared to copper. It is believed that drinking water kept in the copper vessel boosts our immunity. This is why one should use water kept in copper vessel to rinse before any puja. It is believed that this will help get rid of any kind of illness and improves blood circulation.
It is believed that this even improves concentration and helps us focus more on puja. Iron, steel and aluminium are considered impure metals. Hence, vessels made of these metals are not used in religious purpose.