This is done to allow for money to be added to other money. Without that piece of code, this would cause a compiler error, "Operator '+' cannot be applied to operands of type 'Money' and 'int'"
Money money = new Money(5.35m);
decimal net = money + 6;
With the casting operator present it allows these types of conversions to be made without throwing an exception. It can assist in readability and allow for polymorphism where different currencies could implement their own types of casts for example.