Its a Strategical option, and only happens when a team has Forward dominance within the 22.
And they will only do it when they are more than 2 converted trys away from leading, or when they are safely ahead in points.
It is an attacking option. A platform to launch a set move.
When the scrum is set within the 22, 8 players of the defence are bound to the scrum meaning there are less defensive players in the back line.
The more times you set the scrum at a close range the more the intensive the pressure is and as a defender the more resolve you have in your defence of the line. This ususually brings the defence closer to the scrum, aloallowing gaps or an overlap on the out side.
If the the ball is quickly cleared to the backs, there are less players defending with larger gaps between the defenders. It is man on man at a higher pace with less defenders in the equation.
This means the odds of scoring a try is in favour of the attacking side.
There are all sorts of reasons this is the most common. If the question is in refference to what the Italians were doing, They had no intention of playing a set move they were trying to milk a penalty try. It was negative rugby and somthing you dont see often, it was illegal tactics by a very good Italian prop who knew exactly what he was doing, being coached by a very smart South African coach, Nick Mallet.