Comment by Balgair

Because you'd also like to know the phase of the waves. If you get unlucky and get just DC offset of the waves (think of it as the 0 crossing of a sine wave) then you've no idea what the phase is. The peak could be before the 0 crossing and the minimum could be afterwards, or it could be the opposite (pi offset in phase). Granted, this is unlikely to occur, you're much more likely to not run into these scenarios. Ideally, you want 4 pieces of data per wave, the 0 crossings and the max/min, for each wave. From that you can get the amplitude and the phase for certain (again, luck is involved)