For what primary purpose did the spanish enslave many american indians?

The  Spanish conquistadors enslaved American Indians because they needed labour to work in mines and grow sugar.

Slavery in the Spanish American colonies was an economic and social institution which existed throughout the empire of Spain. In its American territories, it initially bound indigenous people and later individuals of African origin.  

The first speech in the Americas for the universality of human rights and against the abuses of slavery was also given on Hispaniola, a mere nineteen years after the first contact. Resistance to Amerindian captivity in the Spanish colonies produced the first modern debates over race and the legitimacy of slavery.    

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small european communities with shared traditions consolidated into large new states (apex)