Most of my projects were in the category cool, but useless;) I programmed sorting algorithms, made some text-based games, a random poem generator, a banking system, a mastermind solver... I think I never actually made a finished product with a nice user interface. The coming week, I'm using Java to make Android games. Right now, the main benefits I see with Java is
1: It can be used to make games, both for computers and phones.
2: If you know Java, a lot of other languages are very easy to learn because they use the same language structure.