You may wish to specify which colonies you are referring to. In general, though, Spain founded colonies in order to extract resources from the ground (especially gold), convert the populations to Christianity, and achieve renown through the act of conquest.
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Spain started colonies to gain more land for itself.
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Yes more land but the main reason was money. Gold, silver and other valuables could be gotten from the locals and shipped back to Spain. Even after most of the gold was looted from the locals, er I mean traded for, the New World had many other resources that could be exploited for profit. But at first it was all gold baby.
Spain created colonies to bring back wealth (gold and silver) from the New World to the Old World.