It's fairly easy to setup data binding with setters/getters, but quite often you don't want to give the developer write-access to your properties. Flex Builder will throw a warning if you don't have both a setter and a getter and you're specifying the property to be bindable.
The solution is to manually fire an event when the property changes. This event then causes Flex to fire the binding event that causes a property update.
private var _myProperty:String = "";
 
[Bindable (event="myPropertyChange")]
public function get myProperty():String {
return _myProperty;
}
 
public function doSomething() {
_myProperty = "New string";
dispatchEvent(new Event("myPropertyChange"));
}
Generally, doSomething() will get called from somewhere else in your code, and your view (or whatever) will bind to the read-only getter.