[smcmurtry]

The numbers track the order that the cells were executed in. So, strictly speaking, you need the numbers to be able to reproduce the output of the notebook's author. However, I think it's bad practice and pretty uncommon to publish a notebook where you need to execute the cells out of order. So most of the time the numbers aren't useful.

[analog31]

Indeed, and the line numbers are a warning that you might have an order-of-execution bug lurking somewhere.