What do you use layers in Illustrator CS3 for?

Answer 1

The short answer is "Layers in illustrator, like layers in all graphics programs, are a convenient tool to help designers organize their visual information"

For a while I wondered why anyone used layers in Illustator at all, since all of the vector shapes you draw maintain their integrity in space and can easily be 'brought to front' or 'sent to back' etc. Layers seemed a bit useless to me at first.

But as you create more complicated illustrations, it becomes invaluable... separating elements onto layers allows you the option to lock them. When a layer is locked, you can grab and edit foreground elements without disturbing the background. Or vice versa. Also, it's fantastically handy for locking an image layer when you're doing a vector trace. (If you've seen A Scanner Darkly, or perhaps Waking Life, or those bank commercials that are rotoscoped, you know what I mean).