Since we are talking DC, you have V=IR, or more to the point, I=V/R. R is the glow plug resistance, which should be constant (more on that in a moment). V is what you adjust with the knob, it does change the voltage of the output. I (the amperage) is what the needle shows you, it's telling you what the result of turning the knob to a particular setting is. So, you adjust the voltage which gives you a resulting current. Knowing the current produced for a specific voltage is more useful than just knowing what voltage you set the knob to. The same knob position always results in the same voltage. (in theory anyway)

Now, R (the plug) should also be pretty constant. If it changes, it can tell you things. It can tell you that your plug is going bad, or if the engine is flooded or not primed, etc. In general, the knob setting should result in the same needle reading particular plug all the time, if it doesn't, then something is happening that you might want to be aware of.

Personally, I went to a cordless nistarter thingie a while back for convenience, so I don't remember exactly what changes to the amperage mean anymore, other than the obvious readings of "nothing" and "everything".