The point is that a miner can start work on the next block very quickly without downloading everything. This was the major concern of miners especially in low bandwidth countries like China.
If they had to download 2 to 8 meg before starting a new block they are at a disadvantage. It also stops the need for the centralised network used by miners currently for block propagation between pools.
The Real-time Bitcoin Gold Exchange and API
Combining the historical security of gold with the speed and transparency of the bitcoin blockchain