It's for the guidance system, it only pitches on one axis so they roll around to the correct heading before they pitch over. On the older Soyuz variants they ph

It's for the guidance system, it only pitches on one axis so they roll around to the correct heading before they pitch over. On the older Soyuz variants they physically rotated the launch pad. This is because the engine gimbal better in certain directions due to the design. I'm not sure if other rockets do this, but I'd expect the Atlas V does due to the engine configuration. The Space Shuttle also did, but for aerodynamic reasons.