Way back in the old days we called the chord that Scorpio describes the "long A" to distinguish it from the standard "short A" (002220) that most folk guitarists played. One advantage of a long A was the distinctive sound of the three tonic notes, especially the high a note (1st string, 5th fret). Another advantage was the ease with which you could quickly change between A, Amaj7, A7, A6, and A modal (the power chord Peace describes), just by moving your pinky while maintaining the open 5th and 6th strings for bass. That's why blues musicians love it.