Anachrocosmic is a standard roleplaying game with some not-so-standard rules. True, you have players and an optional GM, you roll dice to determine outcomes, and you have stats to keep track of, but this is not a game about developing a character and making moral decisions. This is a game about combat and blowing stuff up... with style.
A game master is used if the group wants to focus more on roleplaying than using the rules primarily as a combat simulator. The game setting is well developed.