The White Rock based half of the cross are good layers, so they provide lots of cheap eggs. There are also probably fast maturation Genes from this side. The Cornish Rooster provides the heavy muscle genetics. If a Cornish hen was used, the eggs(and chicks) would be more expensive.
Actually, I suspect modern X's use at least two generations(F2) of hybrids
There would be 4 sets of inbred lines. Two would be crossed to provide the Cornish rooster and the other two the Rock hens. Hatcheries buy 1st generation cross roosters and hens for breeding stock and incubate the 2nd generation eggs.
The result is lots of eggs with genetics to eat nonstop, grow like crazy, and put on lots of muscle with(inadvertently) little ability to reproduce or "think"!