1) To fool the untrained so they can sell their replica for a profit. 2) So people (and museums) can have something to display or fill a hole in a coin album or to be able to display a replica rather than an extremely valuable and irreplaceable original. The US Mint has made replicas of some of their own rare coins. A common replica was an electrotype which was typically made using electricity and created very good copies. Your coins appears to have been plated and the plating is coming off in places. That points to it not being a geniune coin.