Brassicas are grown as a forage crop for livestock to eat, including kale, grazing turnips, stubble turnips and rape/kale hybrids are suitable for feeding to most cattle and sheep, it fills that gap in winter, early spring when the grass isn't growing and farmers don't want to buy in feed.
That photo is a little unusual, but there are other reasons why a crop is better eaten by livestock rather then harvested for the plate. The price being paid for the crop, or an infestation, or hard frosts (that we can't see here) that made it unsaleable being three. And if it was planted as feed any choice is informed by the price of the plant at the time of the planting.