Singapore is the sort of city that makes a one-way ticket feel sensible. You can land at Changi, spend a few days exploring hawker centres and neighborhoods, then decide whether the next stop is Kuala Lumpur, Bangkok or somewhere farther away. The route is flexible. The paperwork deserves a little more structure.
Singapore's Immigration & Checkpoints Authority (ICA) says short-term visitors should have proof of onward travel, along with sufficient funds and any visas needed for their next destination. That makes a one-way arrival possible to plan, but it is not a reason to leave the departure question unanswered.
The useful distinction is between buying a round trip to your original city and showing a credible plan to continue your journey. Those are not always the same thing.