They are just different. I think that when the KR were alive and fighting in the Desolations or just teaching, they had different jobs. Windrunners would probably be more like scouts, due to the fact that they can potentially fly above enemies to spy. Whatever order Dalinar is would probably be like the guardians/front attackers if they really get enhanced Shardplate abilities like some people have theorized about Dalinar. And Soulcasters would be the people who would stay in the back, potentially target leaders with their distance attack, and supply food and shelter to the army/organization. This how I think the KR worked when they existed.