Layers are awesome.
They are the key to everything visibility in AutoCAD.
Because of that, people generally feel that for maximum control, you need maximum layers. Compound that with the fact that a drawing rarely stands alone. In fact most drawings have other drawings with maximum layers in them being referenced in. The result... (infinity)x(infinity) layers.
Well, not infinity, but in the example below, the user needs to navigate 233.
By selecting "Xref" in the left panel, then checking the "Invert filter" box, the user only has to navigate the 98 layers that are actually in the drawing being used. That helps!