A thought just then occurred to me I'd never thought before.
While reality is more nuanced, the sound-bite version of the story I use is that sharpening (with a stone or belt grinder or wheel or whatever) removes metal and creates a new edge, while a honing rod aligns the teeth of the existing edge to reset it without removing metal.
I cleaned up my Idahone ceramic rod this afternoon with a bit of BKF to remove the gray marks from the rod.
It just occurred to me - that gray is in fact metal transferred from the knife to the rod, right? So I am taking off a minute amount of metal? Which means, among other things, I need to pay much closer attention to the angle I hold the knife against the rod. (It's close to what I sharpen at, but I'm not nearly as careful.)