Three possible reasons why European countries wanted colonies:
- Economics. European countries felt that they needed colonies for two reasons having to do with economics. First, they felt that colonies would give them a source of raw materials that they needed for industry. Second, they felt that the colonies would be a captive audience for their manufactured goods.
- Prestige. European countries did not want to fall behind in the race to be seen as major powers. They felt that they would look more powerful if they had colonies and they feared that they would look weak and unimportant if they lacked colonies.
- Military power. Possession of colonies would give countries the ability to project their power around the world more effectively. It would give them bases for ships and soldiers far away from their own homeland. This would give the European countries more military power far from home.
The European countries tried to get colonies for some combination of these three reasons.
I would say that the main reason is money. By establishing colonies, the European countries have another market to sell their manufactured goods, thus increasing their wealth.
Another reason is slightly related to the first. The European monarchs believed that there was a lot of bullion, gold, and silver lurking in the colonies. They wanted to plunder those resources for profit. In fact, the entire purpose of the colonies was for the benefit of the mother country (something called mercantilism).
The last reason is power. The European monarchs consistently fought with each other over land. Why? This would make them seem more powerful. By controlling more colonies, the monarchs could prove that they were worthy of respect.