Most operating system kernels are optimized for generalized use. While this does not preclude operation as a server, we have to change a few settings to fully utilize our available hardware. This isn't simply a series of configuration modifications meant to increase performance, but critical kernel-related tweaks meant to prevent outages.
Though, while we're on the subject, there's no reason to not include purely performance-enhancing changes. Getting the most out of our hardware prevents unnecessary operating strain on existing resources. A server running too close to its limits cannot be considered highly-available; an unexpected increase in demand can render a server unusable under the right circumstances.