Multiple computers is to be able to run different things concurrently. Sometimes possible with VMWare on a single computer as longas the machine is powerful enough. However, VMWare can't always help out with all the hardware we tend to need.

Multiple monitors is to allow you to view a lot of information at the same time. Most IDE for software development consumes a lot of screen space. Then you need your datasheets available. And then you need info about yuor subject under test. If developing a PC program, you need to show that application somewhere. If developing an embedded program, then you may have the debugger inside the IDE but instead have one or more serial consoles, to access a monitor inside the device, or a PC oscilloscope showing signals on the PC.

I have for a number of years been using four computers and two monitors connected to a 8-to-2 KVM. I'm currently looking for a new solution since the KVM is analog and I'm in need of something similar for DVI-D signals. Right now, the main machine uses DVI inputs on the two monitors while the other machines are using DVD-A/VGA to the KVM and requiring me to both make a selection on the KVM and to switch video input on one or both monitors.

But in the end, you can hardly get too much screen space available. And since so many software developers are running with multiple monitors, they don't see the problem when their tools requires the end user to also make use of multiple monitors.