This has nothing to do with the subject under discussion. But since you ask, it is all about the finished work of Christ. The field is the world and the treasure is the saints of God. The Church is the purchased possession of Christ, and Christ gave His life and shed His blood for it.Why did He need to buy the field?
And they shall be mine, saith the LORD of hosts, in that day when I make up my jewels; and I will spare them, as a man spareth his own son that serveth him.(Malachi 3:17)