(I'm new to Inkscape, but I have a little experience with Gimp and layers.)

I vectorized an image in Inkscape the other day (with the Path - Trace Bitmap process). I ungrouped the layers and pulled them apart. So I could see all 4 layers. I tried this again today, with 15 layers. But regardless of weather they're grouped or ungrouped, if I opened the layers window/dialog it doesn't show all the layers - It only shows 1 layer. Why does it only show 1 layer?

My guess is you have run the trace with single scan mode even though you have set 15 scans in the other box.

On a side note the trace bitmap with multiple scans results in a group of paths, not layers as in gimp.
Furthermore the svg specs doesn't support layers; inkscape uses special groups to implement them.
For most use cases adding new layers only complicates the drawing process.

There are layers in Inkscape somewhat like in GIMP, but objects can also overlap in a manner called z-order. The scan will stack objects in z-order.

http://tavmjong.free.fr/INKSCAPE/MANUAL ... Order.html

the trace bitmap with multiple scans results in a group of paths, not layers as in gimp.

Oh yeah, it does say that in the trace bitmap dialog - "Multiple scans: creates a group of paths".

There are layers in Inkscape somewhat like in GIMP, but objects can also overlap in a manner called z-order. The scan will stack objects in z-order.

Yeah i watched a video tutorial on Youtube that talked about the z-order. I would have thought objects overlapping like that would also be in layers, but apparently not. Which makes me wonder what people use layers for in Inkscape.

Are layers useful for anything?

Layers are useful for many things! I make heavy use of layers, but I've heard some people say they almost never use them.

I almost never use them, use alot of grouping instead.

But they can come handy if you work with something heavy on the rendering, where you can switch off the too complicated parts temporarily.

I use layers for hiding/showing objects... foreground, background, text, variants, etc.