The advantage of a generic rule system is that players only need to buy and learn from one main rule book, saving money and time. Since most settings share a large set of features, such as characters that can move and fight, players would not have to re-learn such basics when starting in a new setting using those generic rules. This eases players in the move from one setting to another. The players and the game master choose the desired setting before starting a game.
Generic game systems also have their share of disadvantages. Often, the basic rules of a generic system are more complex than those designed for a specific setting. Generic rule books have to cover features and aspects that might be of little use in some settings. Also, game developers still might need to produce rule books designed for specific settings to provide meaningful gameplay. If the developers take this too far, it can offset the original advantages of a generic system in the first place.