Post 1984 pennies are composed of a zinc core and a copper shell. Why is this best?
In what currency........?
Most of the metals we use are alloys. This is certainly true for coinage. Clearly, the alloys used for coinage must be: (i) durable; (ii) resistant to chemical and physical damage; and (iii) easy to machine. This particular alloy evidently meets these properties. If there is a coated zinc core, presumably the less reactive copper coating hinders corrosion of the zinc.
See this old answer for more examples of alloys.
The alloys we use for currency must also be cheap and inexpensive. We are long past the days of