I mean if you have multipe APs (eg: Mesh solution), having the ONR can be an advantage if you only care for wireless performance. Basically the main advantage of ONR is to save one router (eg: in the DB box) as it is a router.
Just an example, for new BTO HDB, if you only care about wireless performance, and you have 3-nodes of TP-Link Deco X20.
ONR user -- you can put the ONR in the DB box, 3 nodes in the living room, master room and bedroom.
ONT user -- you may have to put one node in the DB box, and you may need to add one switch in the DB box, then one node in the living room and one in master room, resulting in less coverage compare to the first solution.
But to me it is worth to have an aditional router in the DB box than using the lousy Singtel ONR as the main router.
I do understand many users may be fine with Singtel ONR and the Singtel provided wireless router/AP since they only do simple web browsing and media streaming, and they may not know the limitations. But you do lose the choice. So for the new sign-up, I will recommend to avoid Singtel (except the niche Singtel 1+1 plan, only for niche use cases). Now you are in this forum, hopefully at least you know the limitations of Singtel ONR.
For example, I will recommend many users here to use alternative DNS and not to use ISP DNS. I recommend using Cloudflare Family Protect DNS: 1.1.1.2 (malware protection) or 1.1.1.3 (malware and adult content protection). You can not do that with Singtel ONR -- you have to set it up per device.
Ref:
https://blog.cloudflare.com/introducing-1-1-1-1-for-families/
There are also privacy/security concerns about the Singtel ONR as it is remotely managed by Singtel.