The main reason spain wanted to colonize the americas was to

The main reason according them was to spread Christianity. This was the goal that they emphasized  very strongly.

It has been said there were actually three major reasons summarized as "Gold, God and Glory "

Gold meant that one goal of Spanish colonization was to find gold or otherwise get economic benefit from the colonies.

God meant  a goal to spread Christianity.

Glory meant a goal that both individual  explorers and their country of origin wanted to gain glory and power.