What does self._compute_output_and_mask_jointly = True do in tf.keras.layers.Masking layer?

Question

tf.keras.layers.Masking layer has _compute_output_and_mask_jointly set to True in its __init__(...), what does this attribute do other than telling what it is doing in its call(...)?

def __init__(self, mask_value=0., **kwargs):
  ...
  self._compute_output_and_mask_jointly = True

In addition, the mask has been created and applied in call(...). What is the purpose of compute_mask(...)? Seems redundant.

  def compute_mask(self, inputs, mask=None):
    return tf.reduce_any(tf.not_equal(inputs, self.mask_value), axis=-1)

  def call(self, inputs):
    boolean_mask = tf.reduce_any(
        tf.not_equal(inputs, self.mask_value), axis=-1, keepdims=True)
    outputs = inputs * tf.cast(boolean_mask, inputs.dtype)
    # Compute the mask and outputs simultaneously.
    outputs._keras_mask = tf.squeeze(boolean_mask, axis=-1)  # pylint: disable=protected-access
    return outputs