Why did God send Hagar back to Sarai in the desert?
God sent Hagar back to Sarai because He operates with the big picture in mind, not merely the present. Returning her preserved the unfolding account so that eighteen years later she and Ishmael would go out into that same desert, providing Paul a lesson for believers today.
Les teaches that after Hagar conceived Ishmael, she taunted Sarai, and Sarai kicked her out into the desert, whereupon the Lord sent her back. The reason is that God always has the big picture in His mind. He does not necessarily look at the here and now; He can leap thousands of years as though it is a day, and that is what He did here. Eighteen years later Hagar would go back out into that same desert, so God was programming the whole scenario so that Paul could use it as a tremendous lesson for believers today. None of this comes by accident. Later, when Isaac was five years old and weaned and Ishmael was about eighteen, that same attitude of rebellion and taunting came in, and Sarah saw Ishmael mocking.