Parent plants are much more valuable than the GMO seeds farmers buy. A farmer who plants a cross-bred GMO corn crop could keep the resulting seeds and re-plant them, if she wanted. (I mean technically she could, because the seeds aren't sterile, as is often alleged, but she would likely face legal repercussions.) However, a crop grown from cross-bred seeds will contain a mix of corn types, most them inferior in quality. Parent plants, on the other hand, breed true generation after generation, carrying the traits companies engineered into them. The sequences of parent plants' genes represent some of the companies' most important intellectual property.