It is really the SIMPLEST way to create REAL apps for Android. Other solutions do exist but the code is not as clear with B4A and their app's weight is heavier. Not mentioning the ability to have a real control on what you have produced.
And as you asked : when you have understood how to code with B4A then you can easily convert many projects to B4J. And B4J is able to produce code that you can execute on OS X, Windows, Linux, etc. It's worth the price really.