Do I need an ONT if I have a router?

I'm curious, do I really need an Optical Network Terminal (ONT) if I already have a router set up in my home? I've heard that ONTs are often used with fiber optic internet connections, but my current setup seems to be working fine without one. Is there any advantage to having an ONT in addition to my router, or is it just an unnecessary expense? I'd appreciate any insights you might have on this topic.