It's designed to be used to feel inside a lock that you've already set some pins on but not all--the reason it's perfectly flat is so you can feel the pins to see which ones are set, binding, etc. while moving them as little as possible so as to not accidentally unset any pin stacks that are set.
I think.
What they're ACTUALLY good for (and that I use my Falle-Safe #1 for that gordon mentioned) is setting the check pin on a Schlage Everest after the other pins have been set--it's actually quite good at this, better than a hook or anything else I've found (and I'll give credit where it's due--it was zeke that put me onto this). And it's good to have just for this because that check pin in the Everest can require quite a bit of force to set, especially if it's the last pin to bind in the picking order as I suspect it tends to be (it is on mine), because you could definitely end up gouging any hook you try to stick in there because of the force required and the sharp warding. Most hooks probably wouldn't fit anyway since the angle would be too steep.