The advantage of "you attack first" is your opponent's respond can be a much smaller set then if you allow your opponent to attack first.
When your opponent attack first, his first attack can be a kick, a flying side kick, a flying knee, a punch, a superman punch, a single leg shoot, a double legs shoot, ...
If you don't have experience to deal with a flying side kick, when your opponent does that to you, he will lead you into an area that you are not familiar with.
When you kick/sweep at your opponent's leading leg, at that moment, his respond is limited. You want to lead the fight toward the area that you are more familiar with than your opponent does. That will be your advantage.