what is "population" count info good for?

[Taqwus]

If you want pop growth, you should take a positive Growth scale, and it'll still be slow. Oh, and back off the taxes in provinces where unrest is an ongoing problem due to e.g. magical sites so your patrollers need to kill fewer people each turn.
I haven't checked whether magic can be used to affect this; the most likely suspects would be Gift of Nature's Bounty (for boosting agriculture), Gift of Health and Well of Misery (these two for reducing mortality rates). GoH I cast often enough to figure that it's probably not giving much of a pop growth boost if any.
If you really want to see more dramatic pop growth, you could mod the effect of the scale, IIRC; see Alex Poger's scale mod for an example. Generally speaking it's far easier to kill pop than it is to get it back.