Anyone who wants to sign any kind of contract in Spain, whether it's a rental agreement, internet, bank account, cell phone, car purchase, etc., needs a NIE, which is also their tax number. This does NOT automatically make you liable for tax in Spain.
You need it constantly in everyday life, even when accepting a package, for example. So there's really no way around it if you want to live here or stay for a longer period of time.
You will be given your NIE number directly at the appointment at the office. It is a white sheet of paper with a number on it and must be kept safe, as the authorities always want to see the original; a copy is of little use.
Link to service