For expansion, or for peoples modded nations to slip into.
If you turn on 75 thru 90 thru command added to the map, or by command switch, then they arent quite empty. They have very basic starting armies, a castle, etc and do act as AIs. Mostly just "fill material" waiting for someone to put in something better. They flags those nations default to also give the impression of being Kristoffers playground. They arent all the same. There are slight differences, But they arent ALL different. There are many that use the same flag.
If you check my mod page Ive got some that show me playing around with them. Just filling them with minor nations so that the SingleAge mod fills up more of the giant maps I like to use
http://www.dom3minions.com/mods.htm