I think the purpose is just to get you ready for the parade. so for like the people that are at like paradise pier and dont know the parade stops and dont think its coming, the green army men are like a reassurrance that the show is on its way.
Saw it and loved it. It excellently gets every pumped up for the Block Party Bash, as the Green Army Men get people to come out of their 'stand and stare' shells to interact with the parade. Without it, I imagine people not knowing otherwise might not know to what level they can/are supposed to participate in the parade. It's like a cool icebreaker, and the Army Man did a superb job the last time I saw.