Rucking question

Question
I have a question about the ruck. When I watch rugby matches of national teams, I see that after the ruck forms, the team that is on the defensive will attack the ruck (tackle players in it, try and lift players off the ruck, etc).  I have always wondered why this is, and have never found a satisfactory answer.  I would greatly appreciate your advice! Thank you!

Answer
Basically, its to get the ball back to your side, the fast you get there and the harder you hit, the better the chance of getting he ball back.

Another reason is to intimidate the other players, so if you keep on going into the rucks etc really fast and hitting hard, hopefully the opposition will not commit players.