Rule number 1: There is never enough processing power! For a specific application, a processor may be fully capable, but as more processing power becomes available, different and new uses will fully consume and overwhelm the processing power available.
1. Predictive and focus tracking
2. Evaluative metering.
3. Live view processing
4. Advanced frame rates.
The list goes on and on!
***UPDATE***
The D810 is better than the D800 twins in a lot of ways, but mostly because the xSpeed4 processor provided the power for better focusing, focus tracking, higher frame rates, etc.
And the PlayStation 3 was so good because of it's processor. Because Sony had an exclusive deal for the processor, the Military had to wait until the Playstation 3 was release, bought a shitload of them and then ripped the processors out to do product development on sensor systems.