What the Bible says about Mixed Fiber
(From Forerunner Commentary)
These three commands have the same general thought behind them. They move from concrete to intention, that is, from letter to spirit. The one regarding cloth is easiest to illustrate this principle. The specific application involves linen and wool, and Israel was not to mix the fibers from a vegetable product and an animal product into the same piece of material. It expands in its practical application when we understand that mixing animal and vegetable fibers makes poor quality material. Therefore, the intent—its spirit—is to teach us to purchase the best quality that we can afford.
The same holds true with the other laws that appear in this context. God is instructing us that the best thing to do is to keep the breeds pure and not let hybrids develop. The pure breeds of a species are always considered to be of higher quality.
Why was it done this way? It has to do with the Old Covenant and God's intention in His use of the children of Israel. It was not God's intention at the time to save them. He was producing a historical record so that the church of God, whom He did intend to save, could understand things much more clearly—His purpose—and make proper applications of those laws, examples, and principles to their lives.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part Eighteen)