Describe some work scenarios for which one PC on the table is not enough

One computer is on the general network, another is on the secure/air-gapped network.

OS kernel development on one, with the second computer connected to the serial port (or moral equivalent) for diagnostics.

One machine is dedicated to benchmarking, while the other is where you have a web browser with 14 open windows and 132 tabs, and a video call going, all causing jitter.

I've had a job where my client sent me a computer to use for their projects, and only their projects.

Many forms of hardware or low-level systems software development and integration. Work with regulatory or security type divides needing separate networks and computers. Sometimes where there can be major dedicated workstations and/or displays for single applications - CAD, Digital art/animation, industrial controls, etc.

I have two computers on my desk, a mac for admin, meetings, MS office, and a linux laptop for development work.