Conductors (drum majors) are absolutely vital in marching bands. Marching bands (both traditional military style and modern style) use formations so large that the speed of sound is a significant factor - trying to keep tempo based on listening to e.g. drums simply won't work, because different positions will hear the beats at different times (and that's not even considering issues like echoes, or the auditory dead zones that closed structures have). Having visual cues for keeping time is a necessity.