Moved to technical.
Layers are whatever works for you and gives you the results you're after.
They can be useful for putting detail in the foreground without affecting background. This could allow you to try different things in the foreground, or to make changes easily in the background. You can use them to sketch in reference lines overtop your main work area, then fade them out so they dont interfere while you're painting.
Adding a layer lets you treat ealier paint as 'dry' to paint over, but you can go back to the lower layer and paint in the wet paint if you want.
You can use a layer to give yourself a mixing palette which you can easily remove from your finished canvas.
You can use a layer to apply a tint to the whole painting by choosing a paper colour, make the paper semi-transparent, and then use the layer opacity control to affect the amount of colour overlayed.
And as ArtRage matures and gets more features, you'll find many many uses for layers.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"