The vast majority is grown on marginal land, just above pasture. They can't grow better crops without massive works of engineering and tons more fertilizer and energy use. The alternative is to just use slightly less of that land, because the animals are going to have to replace that feed from somewhere. Distillers grains are valuable because the fat and protein are used for finishing cattle for human consumption in feedlots so the sugars are either going to the cows or the biofuels.

The "limited value" isn't so limited when we're talking about an additive to gasoline. The first thing we tried polluted the entire world with a background level of lead!

Of course, it also destroys the topsoil without careful management