The term "use it" was introduced as there was a time-factor introduced to speed up the game, and prevent teams from deliberately slowing down the game. Since players don't have watches on the field, and don't want to get confused with counting "one-mississipi", "two-mississipi", the referee informs them by using the term "use it", before they lose possession of the ball. Otherwise we go back to the situation where the ball is kept in the ruck, while teams have more than enough time to get their defensive stuctures ready.
Perhaps there is a lack of knowledge/understanding by the OP of the history of the game, and how laws were developed to what they are now, and how referees are being given directives on certain issues of play?