In java why do I need to add a return centimeters;? My professor Didn't explain at all and ignored me lol.

Here's the code:

public class ha { public static double calcFeetAndInchesToCentimeteres(double feet, double inches) { if((feet < 0 || ((inches <0) && (inches >12)))) { return -1; } double centimeters = (feet*12) * 2.54; centimeters += inches * 2.54; System.out.println("feet" + feet + inches + centimeters); return centimeters; } }