1.3B page views per month – it’s all about performance

I'm interested in why such low CPU usage is a good thing on the app servers. I understand the need for handling spikes, but they're saying in the infographic that max is 12%.

My uninformed, initial thinking was "That's a lot of wasted CPU" on the app machines. Can anyone explain why spending the extra money and having 88%-95% unused CPU vs. scaling back the number of machines and running them at 50% load?