In Chile you need a passport number (or R.U.T. registro único tributario for residents) even when booking a ferry, a bus ride or you name it..... Suppose it is to match the identity 100% and you know South Americans love administrative matters and paperwork in an endearing way :-)

For air travel it avoids you have to insert A.P.I. data after your booking online or at check in like with European or Asian airlines. Look at it as a convenience.