You can use it for softening, or if you plug it in before the color ramp, you can use it to make the edge between shadow and base less mechanical while maintaining the sharp contrast, which is what I usually do 
either way works.
Another thing you’ll often encounter is three different lights of different colors- pure red, green, and blue, used to drive individual masks (see below)
The really interesting thing about NPR shading- at least this type- is that pretty much everything goes into the same color ramp eventually. The interest and complexity comes not from textures but from shading boundaries.
Let me make that more concrete. Everything that goes into this kind of shading can be expressed as a black and white mask before any color is added. In your case, you could toss in some Greater Than nodes to see the boundaries you’ve defined with a Color ramp.
Those masks might be used to add lines (see my sketchbook), add or remove shadows, distinguish boundaries, or do even wilder things (my general purpose shader has a channel just for determining the strength of detail on hair highlights)- but everything is done in black and white before color is ever added. Another great example is this: Cel-Shading Character Studio Progress (0.1.0 milestone available for beta testing) - #5 by joseph
(That project is on hiatus due to some ethical concerns I have about using someone’s else research for my own work)
I’m self promoting a bit, I apologize, this is my niche, so I have a lot of examples ready to go