It's easier on the computer, all motion detect settings can be applied to the sub-stream, but you can still record the main stream. That's the short answer, I'm sure others will have more detail.
Edit: I have 11 cameras, utilise the sub-stream, and the CPU right now is showing 6%. I'm not sure what the figure would be if all 11 cameras were only main stream, the CPU would go through the roof.