The first impression is that of how difficult it must have been to do all this. Going out to the very edge of the far-flung Spanish empire, rounding up a group of natives, building the largest and most impressive buildings in the region, teaching the natives about Christianity, teaching them practical trades, teaching them to live in settled communities - it all sounds impossible, but it was done - many times.
There are two aspects to these enterprises: the financial, and the spiritual. Taking the financial first, the bills for all this were paid by the kings of Spain. Of course the Spaniards were primarily interested in exploration, and in finding gold and silver. They devoted a lot of effort to this, but they also had a goal of converting the heathens. The kings were fairly consistent in this, and there were limits to what they were willing to spend, but it remained part of their program throughout the mission period.
Of course it is true that the presence of missions bolstered Spain's claim to these large areas, otherwise devoid of European presence, but the Spanish crown's commitment to conversion of the natives was as consistent as any aspect of their policy.
The other aspect of mission-building is perhaps more interesting. The Franciscans and the Jesuits who actually managed the work were dedicated, well-trained, unselfish and competent. To them, conversion of the Indians was a calling directly from God, to which they responded with enthusiasm. They had true grit.
Most, if not all, were born and educated in Spain, and apparently their fondest wish was to travel to the ends of the known world and save the heathens there. Missions had already been succesfully established in Mexico, so there were models to work from. This helped some, but probably not much.