Now you're fighting with teammates for kills, etc.. (sirlin.net)
On some missions you don't even get a choice in this because the story line splits putting each pair of teammates in different areas from the start. (andymerrill.com)
You can use it to help make more attacking options like giving time for teammates with overlapping runs or use it from a defensive standpoint like stopping a rushing attacker getting to the ball. (fifasoccerblog.com)