Well, flash in its current form is better at certain tasks than either hard disks or RAM. Hard disks are big, cheap, and slow. RAM is expensive, power hungry, and fast. If I remember correctly, flash is faster than a hard disk, more energy efficient than either, and way cheaper than RAM. I'm not sure of the implementation details, but you could apparently make a system better on all three axes of performance, cost, and energy usage by mixing in flash.