Were colonies useful mostly because they provided a ready market for manufactured goods?

No. European countries wanted colonies because they wanted to gain more wealth. Gold and silver from the colonies made the countries richer. Another reason was for the raw materials that they needed for their factories. Getting raw materials from the colonies was much cheaper than buying from other countries. One more reason was the desire for land and power. Having colonies made replenishing supplies easier and in case of war, the colonies could be used as military bases.