It flows and sounds cool!
Besides this, the ruck is a crucial part of the game of Rugby. And when it comes to rules and regulations, the ruck is one of the more complex parts of rugby union. When a tackled player goes to ground, they must release the ball immediately. As soon as that happens, the opposition will want to get their hands on the ball, and the team in possession will not want to give it away. According to the laws, "the ruck is a phase of play where one or more players from each team, who are on their feet, in physical contact, close around the ball on the ground". So to gain possession, both sides must try to drive over the ball to make it available for their team-mates.
Check out this video to learn more.
We realize there are 5k races with similar names with the purpose of recognizing the men and women who serve in the armed forces and the amount of extremely hard work they endure and effort they put into protecting our great nation by running with a "ruck sack". You are welcome to run with a ruck sack, but it is not necessary.