Only works in province adjacent to castle you own. Collects tax due to you that month. I'd suggest the tax order should be of short duration. Even with a horse, its going to take me at least 32 days to visit all 4 of the immediately adjacent provinces with one noble. Added to this the fact that it would be tedious in the extreme to submit orders to do this every month.
Nobles send out other spods to do that sort of thing.