Plants in the Bible generally represent facts, knowledge that can be gleaned from the world. Plants that can serve animals as food represent facts that have the potential to be put to use, ones that can lead to something good. A pasture, then, is an area that offers a great deal of this sort of useful, life-building knowledge. "Pasture" is often used as a verb in the Bible as well; to pasture animals means teaching people facts that can help them do what is good in life.