Regardless of the sport, replay challenges help to improve the legitimacy of games as well as the overall level of play. Even with several years of training, some events are just too close for an umpire or referee to discern on their own. If they can be assisted by technology, it helps to ensure that the right calls are made. This helps the game to be legitimate in that people cannot blame an outcome on a poor call by a referee. It also raises the level of play in that players have less leeway in what they do.
Of course they are not an unnecesary use of technology. After all, referees are only human beings who of course, are totally entitled to some mistakes and miscalculations, as it often occurs in most sportin events. For instance, as a baseball fan, I am tired of this arbitrary "strike zone"
With massive amounts of the American public consistently tuning in to watch every sporting event, big or small, and the athletes making millions of dollars a year, the use of replays is absolutely necessary. There will always be human error in judgement due to the fact that the average reaction time of a human being is no where near that of a computer or even a camera. With large dollar bonuses often attached to some games, it is necessary to make sure the correct call is always made.
I believe that in a Professional league where refs have to make very difficult calls on a constant basis that there should be a way for a team to challenge a call, being able to view back the play and ensuring that the correct call was made is something that benefits the game in general.