1.The biggest advantage AC has is that you can step up or step down AC voltage relatively easily using a device known as the transformer.2.Transmitting power over large distances requires you to choose a higher voltage level. This has something to do with power system stability.3.Using higher voltage levels helps in reducing power losses on transmission lines since for a fixed amount of power, you require less current.Since DC does not have the capability of stepping up / stepping down voltages until very recent times, AC is preferred.