# Why do we add additional axis in CNN autoencoder while denoising?

When denoising images, authors of tutorial add an additional axis to the data and I cannot find any explanation why... I would appreciate any answer or suggestion :)

x_train = x_train[..., tf.newaxis]
x_test = x_test[..., tf.newaxis]


Then the encoder is built from the following layers:

 self.encoder = tf.keras.Sequential([
layers.Input(shape=(28, 28, 1)),