Emphasize works with the automated colonist assignment/governor to determine what tile/slot to put each colonist.
Warning, it seems that if you emphasize crosses they will get worked over pretty much anything else. You only get 3 levels (normal, emphasize, de-emphasize) but the AI seems to have its own internal rankings as well (it doesn't balance all the emphasize items but assigns them in some order).
It works pretty well and worth setting them up for each colony, along with the trade routes.