Immigration officers might request it or you did not fill in the arrival flight on the arrival form or just to double check you filled out the correct flight number on the arrival card.
Corrected they are actually proper tickets as you can log into the airlines website and actually see things like your seat assignment etc, they are just being cancelled after 24 or 48 hrs. Having said that I dint recommend Flyonwards - I have used them and did not receive my ticket on 1 occasion and needed Paypal to intervene to get my refund.