I’m from the UK and have been asked to provide evidence of the return flight more than once. IIRC it was if my return (or onward) ticket was not part of the itinerary used for check in. I’ve always assumed the Singapore authorities need to know I’m leaving again. I do always make sure I have it ready to be shown in the airport(s) now. Don’t see it as a big deal myself.

(As an aside I have also been required to provide evidence of a return flight when flying to Indonesia)