When starting a render job in programs like Cinema 4d, Vue D'Esprit or Carrara usually all available processors are assigned to the rendering process.
This means that rendering is running at 100% power which normally is exactly what you want because this leads to fastest render times.
But sometimes it is desired to get some cpu power for other tasks.
For example:
A rendering of an animation is currently running in Vue and you are expecting 1-2 days of render time.
In the meantime you want to do a short render of a still image in Carrara and need some processor power for this task.
Read more: CPU Affinity