Good handouts can save students from laborious, sometimes even mindless copying.
They can prompt and stimulate useful student activity during and after the class. They can form the basis of a valuable student file of learning on your course. Good means that it does whichever of these things are important, and is also legible, cleanly and simply well designed, and, again, interesting. Warning The best audio visual aids and handouts in the universe won't rescue a presentation which is weakly-structured, short of appropriate student activities, lacking in a clear story or poorly explained. Such a presentation, even with good AV aids and handouts, will merely be welldecorated.