How does Christianity traditionally explain the need to subject the Israelites to slavery in Egypt?

In Genesis 15:13 God said to Abraham: > "Know for certain that for four hundred > years your descendants will be strangers in a country not their own > and that they will be enslaved and mistreated there." (NIV) These words were spoken around 200 years before Israelites arrived at Egypt. Why was enslavement of Israelites in God's plan in the first place? Why was it necessary for God to put Abraham's descendants through this ordeal? How is this matter explained in RCC, EOC and main-stream Protestantism?