Answer:
See below.
Step-by-step explanation:
That kind of depends on who you ask: the settlers (the real people making this dangerous trek across the ocean), or the government.
Let's think about the settlers themselves first. You're in Spain living your humdrum life and you're hearing all these fabulous tales of gold and silver to be had, or maybe just land to farm in a favorable climate, so whatever you grew you could sell and maybe become rich that way.
Or maybe you lived in a city doing some menial job, paying rent to a landlord, but you dreamed of having your own land and maybe being somebody. Even if you didn't grow a cash crop that would bring you wealth, you could be self-sufficient and no one could tell you what to do.
Or maybe you had religious issues, like the ones that later drove a lot of the emigration to America. But I don't think that was much the case in Catholic Spain.
Or maybe you were a friar or a bishop who wanted to convert those benighted savages to the One True Faith. So you round up some priests to go and do that, building little settlements in the process.
How about the government of Spain? They were promoting settlers to go and inhabit these new lands. Why?
Taxes.
Okay, why else?
Taxes.
Alright, I get it, but anything else?
Sure, exported goods that would be shipped to Spain and......taxed. Maybe taxed on the way out of Mexico or wherever, but definitely taxed on the way in to Spain.
So money has always been a big driver for countries to encourage settlement in their new colonies. People make money there and pay taxes, and the home government gets a lot of that. People are productive there and grow or manufacture goods that get taxed when they're exported and/or imported; the home government gets most of that.
But there's another reason the government would've encouraged settlement, and that's to prove possession. Say there was an island in the Caribbean that Spain claimed (there were many), and one day an English ship full of settlers looking for a place to plant roots in the New World pulls into its natural harbor, looks around, and doesn't see any Spaniards. "Huh, I guess Spain doesn't care too much about this island, so let's take it over."
Some of that happened, though I've over-simplified it. But the saying is that "possession is nine-tenths of the law," and if Spain didn't have settlers there, real people saying, "This land right here, this plot, is mine, and I'm a Spaniard," then those places were at risk of being taken over by others.
And then because church and state were so intertwined in Spain at that time, the monarchs could look super-pious by supporting missions to convert the natives to Catholicism. And they may've been high-minded enough to really believe that was a worthy cause, but they were also pragmatic enough to know that if you "civilize" the savages, then they settle down in towns and cities and......get taxed.
It always, always, comes down to money, always remember that.
Well, money and power. And power can almost always be converted to money. (Think greedy politicians and bribery.)
A lot of this is over-simplification, and there are other reasons, but that should give you a good start on what to research.