Stacking order vs. Layers?

Question

I'm coming from working with layers in Photoshop and understanding how they work in there. Illustrator also has layers, though you are able to also change the stacking order of items on a given layer  by using the object > send to back or forward.
1. Why would you want to send items to the front or back and not just use layers?
2. When do you want to put an item on a new layer vs. just changing its stacking order?
Thanks.

Answer 1

Stan,
The point you are missing is a conceptual problem common among many users who have some experience with Photoshop (or other raster image program), but next to none with programs like Illustrator (drawing programs) or InDesign (page assembly programs).
The key difference is this: Fundamentally, a Photoshop file is, ultimately, one single raster image. An Illustrator (InD, CorelDraw, FreeHand, Canvas, Flash, Firworks, etc., etc.) file is a collection of individual objects.
In object-based programs, all the objects are located somewhere in one big Z-stacking order. Think of Layers in these kinds of programs as just one heirarchal "level" of "grouping" contiguous objects within the stack. There are several other such organizational "levels". For example:
AnchorPoints exist in the one big Z stacking order. Sets of contiguous anchorPoints are "grouped" (connected) into paths.
Sets of contiguous paths are sometimes "grouped" (combined) into compound paths.
Sets of contiguous objects—including paths and/or text objects and/or raster images (each with its own color model, color depth, rotation, scale, DPI)—can be "grouped" into Groups.
Sets of contiguous objects can be "grouped" into (reside on) Layers.
Groups can be nested. You can have Groups inside other Groups.
Layers can be nested. You can have SubLayers inside other Layers.
In Illustrator and some other vector drawing programs, the Layers palette does not just list Layers. It lists every object on each Layer.
All this organizational scheme is useful and important because of that one fundamental difference: These are object based programs. Now compare this to Photoshop. Yes, a Photoshop document can have Layers. But what is a Photoshop Layer really? It's just another raster image of the same color mode, same color depth, and same pixel count as the Layer(s) underneath or above it. The color values of corresponding pixels in any of the channels of each Layer can be mathematically combined to result in the color you see in that pixel location on your monitor and in the final result. But you don't have a stack of independent raster images. Think about it: In Photoshop, you don't have a 100 PPI CMYK image in Layer 1 while you have a 300 PPI RGB image in Layer 2.
But in an object-based program, you can have a stack of any number of raster images, and each one can be scaled independently without loosing any of its pixels. In other words, the pixels in individual raster images on the page can be different sizes from the pixels in other raster images on the same page. Then you can throw live text objects and paths into the mix, as well.
So more elaborate and flexible ways to organize objects is critically important in these kinds of programs. Example:
You set up a Layer to contain the background raster images of a magazine ad. There are several images, and they are independently scaled and positioned, sometimes overlapping. They need to be sorted in Z-order within that Layer. The Layer enables that particular set of objects (raster images) to be treated as if they were one object.
In the same ad, you set up a Layer to contain the body text.
In the same ad, you set up a Layer to contain the photo captions.
In the same ad, you set up a Layer to contain an elaborate vector illustration.
Within that vector illustration are hundreds of individual objects. Those are organized into subLayers and/or Groups. But the parent Illustration Layer lets you turn off visiblilty or lock the whole Illustration.
And so on. So there are counless reasons "why you would want to send items to front or back" or change the stacking order of objects within a specific Layer—because there are countless situations in which different organizational levels make sense in terms of working efficiency. There is no single one-size-fits-all answer to your question. But trust me, all the Z-stack organizational "levels" are important. And they are all standard fare in object-based vector illustration and page layout programs.
JET