Sometimes, you want to create a ruck without going to ground. Remeber, our definition of a ruck is "Two Opposing Players Contesting Over a Ball on the Ground." So, if you put the ball on the ground (without having it roll forward, because that would be a knock on), and you immediately go into contact with the defender over the ball... voila! You have a ruck (and an off-sides line).
So, when would you do this? Well, there are two times when it can be useful. The first is when you've broken free of the defensive line and you're running in space. You're getting tired and no one on your team is nearby to pass to. So, you aim for the fullback and use this technique. Now, not only does your team have time to recover and catch up to you, but the defense has to run all the way behind their fullback in order to join the ruck! Plus, if you keep the fullback engaged in rucking, when one of your teammates picks up the ball, there won't be anyone else back and they should score easily.
The most important thing to remember is that breaking through the opposition into space doesn't (usually) happen very often and it is important to capitalize on it each time we manage to get through!
The other time this is useful is when your forwards are twice as big as the opposition's forwards. This is pretty rare, so I won't go over it too much here, but basically, if you keep doing this to a defense, they really don't have an opportunity to recover or contest the ruck because you're constantly smashing into them. Effective, but unusual and unlikely to happen this year.