Here are some serious reasons why we need multiple machines running the same OS.

1. Millions of page hits a day will tax a PC somewhat. While we could optimize, programmers cost more than computers.
2. Load balancers and failover are a business requirement. By definition, you can't failover from a machine to itself.
3. When we update the code on the live site, it is nice to be able to take half the machines out of service, update them, then cut over and update the rest.
4. The production site is at a nice hosting facility. Developers work at a different site. Latency sucks.
5. When installing experimental software, modules, etc, it is nice to do that in a physically isolated environment. Ditto when testing out things like OS upgrades.

And those are just the reasons why we have multiple Linux machines. There are plenty more that I can think of, but they don't happen to apply to us.