Hardware

It is kinda more efficient tho.
Power consumption per clock doesn't scale linearly.
Splitting a task into two parts and running it on two cores at 2 GHz will consume less power and generate less heat than running it on a single core at 4 GHz.
Its significantly harder to do that programming wise, but performance wise it has its benefits.
It also frees up the main high performance core to focus on more important tasks.