Why did God send Jacob and his family down to Egypt?
God sent them to Egypt to preserve them through Joseph's abundant food and to fulfill His earthly promise to Abraham by making Israel a great nation. There they would multiply into millions, experience slavery, and be prepared for Moses to lead them out, all within God's eternal purpose.
Les explains that God was remembering His promise to Abraham that his offspring would be as numerous as the pebbles of sand, and He told Jacob, "For I will there in Egypt make of you a great nation." This was an earthly promise, not a heavenly one. Joseph was in Egypt with food aplenty, and in God's eternal purposes the Jews would have a population explosion, coming out some 215 years later as 5, 6, 7 million, more numerous than any other single tribe or nation in the Middle East. They had to go down into Egypt, go into slavery, and be ready to come out when Moses led them; it was all part of the divine purpose.