The real glory for conquistadors was gold. Initially, they found some, both in the Caribbean and in Mexico, but never enough to get, like, super rich. Fortunately, or as I will argue, unfortunately, they did find a mountain made of silver. So the Spanish adopted the mita, which the Incas had used to build roads and public buildings, to mine and process that silver. So one-seventh of the adult male Indian population from each district was required to work in the silver mines for a year, being paid only subsistence wages. Now, you might wonder why the Spanish didn't purchase African slaves to work in the mines. They did in Mexico, but in South America, it was cheaper to use indigenous labor.