Why were slaves so important for plantation owners in colonial america? a. the plantation owners could use their slaves to defend their property from european powers. b. slaves could be traded for manufactured goods with the middle and new england colonies. c. slave labor made it possible to grow cash crops such as rice and tobacco on large plantations. d. the number of slaves determined the number of representatives in the colonial legislature. e. slaves manufactured textiles, furniture, glass, and other goods for export to britain.