It's all about volumes of scale. If you save 15% on a process on a standalone server then that saving will likely go unnoticed. However if you have several thou

It's all about volumes of scale. If you save 15% on a process on a standalone server then that saving will likely go unnoticed. However if you have several thousand servers running the same application, you've now just saved yourself 150 servers. Scale that up again to Facebook / Google's / etc scale and the savings are enormous. This is the same reason why Facebook invested into their own PHP compiler and Google created Go.