Element_shape parameter of TensorListGetItem

I am studying TensorListGetItem and in its signature there is an input_arg called element_shape. I do not understand its purpose. The tensor at the given index inside the tensorlist already has a shape, so what does it matter what I give here as an input?
I tried to use different tensors for element_shape and it almost always gave back the correct result. The only time when the system failed was when element_shape was a scalar tensor with a value <> -1.