That way you can make a java api that's like Java: It can run on any java platform.
Half-joking there, but it is interesting how java is becoming a platform of platforms. (Instead of "I can code in java", how about "I can code in Android")
I guess it would be good to write one piece of code that can be an Android app or an iGoogle widget, but I think unless Google releases an easier way to do it, the old maxim applies: separate the logic code from the gui code. You'll have to recode for the Google, but at least you can keep the code the same that does all the work.