While it may seem logical to store a result in a variable just to use it as an argument to some function, sometimes it may be better to pass the result directly to the intended function.
int x = Math.sqrt(81);
int y = Math.pow(x, 2);
int y = Math.pow( Math.sqrt(81), 2);
The variable x does not even have to be declared, and so you can do away with it completely.
This idea becomes even more powerful when applied to objects. Sometimes objects are only instantiated for one-time use. Consider this:
addWindowListener( new WindowAdapter()
public void windowClosing(WindowEvent e)
As you can see, the WindowAdapter object did not require a variable in which to store it. Thus, it is called an 'anonymous' class, as it has no name to reference it by.
These tactics can save time and space, as well make your code more concise.