If you have a mac with discrete High-end GPU, it's very common that when you are watching some live stream and the battery is dying, the quality of the stream might decrease because the system switches from high-end GPU to integrated GPU to save battery.
@Khryl Or if you planning on playing DOTA or League of Legends (which require high end graphics processing), then switching to a more powerful GPU makes sense.
This actually made me realize why my Mac's battery would drop so fast when I play DOTA. I downloaded a software (gfxstatus) which helps you manually change which GPU is in use to save battery life.
How much more power does an average GPU take compared to a CPU? If a power saving technique is to disable the high end GPU, I'd imagine it would at least take 5 times more power...
@Lawliet, this article (http://www.eecs.berkeley.edu/~sangjin/2013/02/12/CPU-GPU-comparison.html) debunks a good myth. I'll quote the relevant part here: "100-1000x speedups are illusions. The authors found that the gap between a single GPU and a single multi-core CPU narrows down to 2.5x on average, after applying extensive optimization for both CPU and GPU implementations"
Another interesting side note is underclocking and overclocking i.e. running the memory and CPU at lower/higher than rated speeds. Underclocking would give you better battery life while overclocking may give you better performance ( be careful about the heat though)