If your provider has implemented it (Comcast is the only one i know of in north america) then Active Queue Management is a huge quality of life improvement that you won't know you were missing unless you already had a router that implements queue management. https://www.cablelabs.com/blog/how-docsis-3-1-reduces-latency-with-active-queue-management

Not buying another modem when the ISP quietly upgrades the CMTS and makes more speed available in your neighborhood.

Yeah I recently switched from cable to fiber (finally available), and prior I was using an old as fuck modem/router that capped at 500Mbps. My internet at fastest was 380. I rarely transfer files over the network, so figured why bother? (I did have Gen1 Google Mesh though to cover dead spots). I had a bit of a shopping splurge when I got fiber. Nothing crazy, just an upgraded mesh and a switch (Why the fuck does Frontier provide an ONT with 8 ethernet ports but only one is active?)

At least in my case, my DOC IS 3.0 modem was having connectivity issues. My neighbor in another apartment had similar issues: dropped connections, slower than expected speeds, etc. Switching to DOCSIS 3.0 modems solved the problem. I guess Comcast upgraded their hardware and it wasn't compatible with my modem anymore