I'm using 4 monitors, two 33" and two vertical 22".
It has nothing to do with knowing how to do shortcuts, but having to do them at all. There are plenty of instances where you may only need to provide input to one window at a time, but see other things, like documentation, output, logs, or a browser window of the page you are editing. Having multiple monitors allows me to greatly reduce the need to switch between windows to reference something or see results of my inputs.
I personally find that if I'm forced to alt-tab between my IDE and documentation, like when I'm working on a laptop, it's incredibly distracting, breaks my flow, and slows down my work significantly.
It has nothing to do with knowing how to do shortcuts, but having to do them at all. There are plenty of instances where you may only need to provide input to one window at a time, but see other things, like documentation, output, logs, or a browser window of the page you are editing. Having multiple monitors allows me to greatly reduce the need to switch between windows to reference something or see results of my inputs.
I personally find that if I'm forced to alt-tab between my IDE and documentation, like when I'm working on a laptop, it's incredibly distracting, breaks my flow, and slows down my work significantly.