People in the south used slaves because their economy was focussed on things like cash crops (tobacco, cotton, etc.) These crops required a lot of work so farmers and plantation owners could not do it by themselves. So by using enslave persons they could harvest more crops, sell more, make more money, and in return boost the southern economy.
Assuming that you want this simplified...


Use the FOIL method and combine like terms:


And your final answer is: 
Answer:
To retake control of the Holy Land
Explanation:
The goal of the Crusades was to take back Jerusalem from Muslim control.