To elaborate, it’s really easy to forge “regular” coins and really attractive to forge high value coins.
For example, the 1 British Pound coin was, before the redesign, widely forged:
As of March 2014 there were an estimated 1,553 million of the original nickel-brass coins in circulation,[6] of which the Royal Mint estimated in 2014 that just over 3% were counterfeit.
Bi-metallic design is mainly used to make the coins harder to counterfeit, to make it more expensive to counterfeit than the coin’s value
To elaborate, it’s really easy to forge “regular” coins and really attractive to forge high value coins.
For example, the 1 British Pound coin was, before the redesign, widely forged:
https://en.wikipedia.org/wiki/One_pound_coin
(Note for any languages that use the comma as a decimal separator: 1,553 million is referring to 1.5 billion)