Why is God on Sarah's side when she asked Abraham to cast out Hagar and Ishmael?

> The child grew and was weaned, and on the day Isaac was weaned Abraham held a great feast. But Sarah saw that the son whom Hagar the Egyptian had borne to Abraham was mocking, and she said to Abraham, “Get rid of that slave woman and her son, for that woman’s son will never share in the inheritance with my son Isaac.” > > The matter distressed Abraham greatly because it concerned his son. **But God said to him, “Do not be so distressed about the boy and your slave woman. Listen to whatever Sarah tells you, because it is through Isaac that your offspring will be reckoned.** I will make the son of the slave into a nation also, because he is your offspring.” *(Genesis 21:8-13, NIV)* According to Genesis 21:12 you can see that God is on Sarah's side to send Hagar and Ishmael away. Is it to realize his promises? We all know that God's plan works sometimes strange. Can somebody maybe elaborate on this please? Because I can't completely understand why God would do something like this.