Given that Jesus can read minds. Why did God need to observe external behavior to know what was in the heart?

Given that Jesus can read minds. Why did God need to observe external behavior to know what was in the heart? In Mark 2:8 it reads > And immediately when Jesus perceived in his spirit that they so > reasoned within themselves, he said unto them, **Why reason ye these > things in your hearts?** and in Deuteronomy 8:2 it reads > And thou shalt remember all the way which the Lord thy God led thee > these forty years in the wilderness, to humble thee, and **to prove > thee, to know what was in thine heart**, whether thou wouldest keep his > commandments, or no.