Portugal was originally only concerned about establishing forts and trading posts so as to protect the sea route to Asia. As time progressed, they began to see the benefit of establishing more permanent settlements. These permanent settlements would result in the expansion of their political influence. The exploration idea of God, Glory, and Gold also fit. The more permanent settlements would allow the Portuguese missionaries to also further expand and share their beliefs with the African Peoples.
This of course depends entirely on the person in question, but generally speaking people who believe in such things would want to come back as animals somewhere at the "top" of the food chain.
It was outlawed in 1804, and it not only declared the colonies free of Britain but it helped to inspire Vermont to abolish slavery in its 1777 state constitution. Hope this helps.
