title: when to use to_categorical in keras
body: <p>Please someone explain when <code>keras.utils.np_utils.to_categorical</code> is to be used?<br>
  I understand that it converts <em>class vector</em> to <em>binary matrix</em>, probably used in <strong>Deep Learning model</strong>.<br>
  But if we still go ahead and use the class vector itself, and probably use <code>model.predict_classes</code> - what is the drawback? </p>