Why do we need "batch_idx" argument for "training_step" method?

[flourish-727]

Hello all. I am a rookie and just started learning Lightning.
I don't really understand the role of "batch_idx" as an argument of the methods like "training_step" or validation_step.
The method does not really use the argument (see this example from official):

def training_step(self, batch, batch_idx):
x, y, z = batch
out = self.encoder(x)
loss = self.loss(out, x)
return loss

See? "batch_idx" goes nowhere!!!
Could someone please give some hints?
Thanks!