My group just played this game for the first time the other day.  It was only a 3 player game sans Nurgle, but we all enjoyed it.  We did misplay a few rules and are eager to give it another shot.  However, the rule book, on the board break down on page 5 clearly points out that a populus region will specify that it is populus by saying it under the region name.  As far as I can tell, that is the only time it is mentioned in the rule book.  What is the purpose of this?  I have not read every single card that came with the game, but in the game we played, not a single card referenced a populus region. 

That was because you played without Nurgle :) Nurgle's threat dial advancement condition refers to Populous regions, which makes them fairly important for Nurgle's plays.

There's a few Old World cards as well who refer to them IIRC, but Nurgle really is the biggest contributor to their importance.

Yes! I am and how could you play without me?

 Populous regions are core to Nurgle's dial advancement. They are the only places he can get tokens from, which also helps Korne predict where enemy followers are likely to be placed. Ruining the populous regions early helps prevent Nurgle Dial advancement.

 Essentially is a keyword for those regions that other things then are tied into.

Nurgle needs this tag most of all as others have mentioned. I think some cards use it as well.

Also, it's a good indicator of the regions that are worth a lot of VP to ruin