Having multiple players by the ball can allow for more offensive opportunities. It doesn't mean that it will create better chances, but if played correctly, having 2 or maybe 3 players behind the ball allows for having more opportunities to score.
Right footed players's crosses and shots naturally curve to the right and back in, and the opposite is true for the left footed players. Now consider having a free kick. having players of opposite dominant foot allows for both curves. It leaves the goalie with less information on how the shot or the cross would travel if there was only one player behind the ball. The defenders also can't prepare with certainty either.
By having multiple players behind the balls you can draw up plays you simply couldn't otherwise.
One of those plays is for the free kick taker to pass the ball to another player behind the ball, and have them take the shot or pass it back for the taker for a shot. This is done when there is not enough space for a shot, by making a pass (although a short one) more space is created and it becomes easier to clear the ball around the opponent's wall. A good and recent example of such play is Tony Kroos's goal game winning goal for Germany against Sweden in the group round of the 2018 World Cup. The free kick spot was far too much on the left and almost impossible to score from. But a small pass of about 4 or 5 feet was enough for Kroos to bend the ball by the defenders and score a goal.
Another possibility is that you can have players fake taking a shot which can make the defenders and the goalie move to the wrong direction, opening up space for another player to immediately take the shot after him.
Similarly you can have one of the player behind the ball go for a run. Defenders can't mark players behind the ball as they have to be 10 yards behind the ball, this is was the wall is formed. So one of the players behind the ball can run away and find himself in some open space for a better option.
Additionally, you can't directly score from all free kicks in soccer. Some kicks are indirect, meaning that if you kick it straight in the net, the goal wouldn't count. In this case passing the ball to another player behind the ball or crossing it for other teammates is necessary.