[6ren]

My thought was that you'll often light from a typical angle, and use familiar (or even stylized) techniques for that specific angle. To put it in extreme terms, you might only know how to draw with that lighting.

For unusual lighting (e.g. uplit), even an excellent artist would have to give it more attention. Similar to drawing a character from an unusual perspective. At any rate, drawing several unusual lighting angles will exercise one's talents more than using the same standard one.

It's literally looking at it in a new light.

[eru]

Since you only need, say, four angles, those four angles will soon become usual for you.

[6ren]

True. I think this is what's bugging me: drawing a lit version implies information about 3D shape; in particular, the artist has to reason/see in 3D. So, in a sense, it is a new method of entering 3D information, that works especially well for traditional 2D art styles.

Why not have the artist enter height directly, instead of shading from several different angles? It seems like less work (because there's less information to input); though possibly doesn't mesh as well with how 2D artists work... whereas shading is part of the tradition.

There's something I'm not getting here (that might lead to a better way of doing it, or not).