Can someone help me understand QOS

So I recently passed my CCNA for R&S and one topic I constantly struggle with is QOS.

I have been told to think about it as a congestion only management solution. If you have a 1gb link all the way out to the internet and you never even saturate that link, is QOS even needed or even utilized. Essentially FIFO because nothing is ever saturated.

I have always thought that if you have the bandwidth, the device will never queue packets, it will just send it out because it has the available bandwidth. Or it will just leave the queue as soon as it's entered it. I have always thought that QOS is only utilized when you have more data than what a link can hold. At which point it will start queuing the packets and sending out higher priority first.

If my thinking is correct, then are consumer or "gaming" routers that advertise gaming priority first through QOS even doing anything as long as you are not saturating your internet bandwidth?

Or is my thinking totally wrong and if so, can someone help me understand.

Thanks