There are two main knock-on effects.
The first is a greater chance of teams not only advancing but winning the group before matchday three.
The other is that teams can be eliminated after two matches - this would not be possible if goal difference was used first.
Under the old format, a team would need to be four points clear of second to secure top spot after two games.
That was rare because it required the other two fixtures of the first two rounds in the group to both be draws.
Now it is possible to be three points clear and have first place in the bag.
It can happen in a few ways.
It requires either two draws, or first place to have beaten the team(s) on three points.
We have already seen it in Group A and Group D.
Mexico won their first two games and sit on six points - three points clear of South Korea, with the Czech Republic and South Africa on one point.
As Mexico have already beaten South Korea they cannot be overtaken if both countries end on six points. So Mexico have won the group.
They know they will play a last-32 tie in Mexico City against a third-placed team.
The same applies to the United States, who are three points ahead of Australia and Paraguay but have beaten both teams
Thus, the US have won the group and will head to Santa Clara to play a third-placed team.
There's a knock-on effect, too.
Mexico effectively have nothing to play for when they meet the Czech Republic next Wednesday, and could opt to rest players.
And that creates an imbalance the Czech Republic could benefit from, though they can only qualify automatically if South Korea lose to South Africa.
But in a tournament where the best third-placed teams go through, it could be important.
The system is also used at the European Championship, where the best third-placed teams advance.
It also impacts the other end of the table.
Haiti, three points behind Scotland, are out because they lost to Steve Clarke's men.
Similarly, Turkey are bottom and three points behind Australia and Paraguay. As they lost to both of those countries they are eliminated.
It means that Turkey v United States in Inglewood on Wednesday is a dead rubber for both teams.
