Assuming I never exceed the bandwidth of my connection to the internet, will enabling and configuring QoS on my router have any positive affect? I'm under the impression that QoS only helps prioritize traffic if there is congestion. If there is ample bandwidth for all traffic, doesn't all traffic flow through unencumbered?
You're right that you shouldn't need QoS if you never hit the capacity of your Internet connection, but are you sure you never hit it?
TCP is designed to transfer data as fast as the network will allow. If one of your devices is downloading a software update from a local CDN node on your ISP's network, it could use all the available bandwidth. If you were trying to do anything else on the Internet connection at the same time, you'd have congestion.