Take for example a game where lots of cars are spawned.. each belongs to the class CCar. When a car is instantiated, it makes a call to
[CCar registerCar:self]
So the CCar class, can make a list of every CCar instantiated.
Let's say the user finishes a level, and wants to remove all cars... you could either:
1- Go through a list of every CCar you created manually, and do whicheverCar.remove();
or
2- Add a removeAllCars method to CCar, which will do that for you when you call [CCar removeAllCars]. I.e. allCars[n].remove();
Or for example, you allow the user to specify a default font size for the whole app, which is loaded and saved at startup. Without the class method, you might have to do something like
fontSize = thisMenu.getParent().fontHandler.getDefaultFontSize();
With the class method, you could get away with [FontHandler getDefaultFontSize].
As for your removeVowels function, you'll find that languages like C# actually have both with certain methods such as toLower or toUpper.
e.g. myString.removeVowels() and String.removeVowels(myString) (in ObjC that would be [String removeVowels:myString]).
In this case the instance likely calls the class method, so both are available. i.e.
public function toLower():String{
return String.toLower();
}
public static function toLower( String inString):String{
//do stuff to string..
return newString;
}
basically, myString.toLower() calls [String toLower:ownValue]
There's no definitive answer, but if you feel like shoving a class method in would improve your code, give it a shot, and bear in mind that a class method will only let you use other class methods/variables.