They work as a field commander, helping your troops fight better and more cohesively. But command units can only work their magic if they have other troops within their effective command radius. The radius depends on the morale of the command unit as well as the type of unit. Command units are specialized units that vary from single men to heavy tanks, and every mission requires that you have at least one commanding unit. Woe unto you as the Russians with your poorly trained large squads of men should your commanders buy the farm.
Life is easier with the better trained German troops that can act independently more effectively.
Once the fighting starts, you have to run, crawl and sneak your troops to victory. But that doesn't mean the enemy isn't doing the same. The game's AI was impressive and often surprised me by sneaking past my troops and taking victory locations I had taken previously. One small unit completely outnumbered and cornered in a room managed to do a lot of damage to my troops before I could neutralize it. I suppose that's what I get for underestimating the enemy.