Some colonists resorted to smuggling because it offered a way they could avoid the restrictions placed on trade by the British. Restrictions were placed on what the colonies could manufacture, whose ships they could use, and most importantly, with whom they could trade.
British merchants wanted American colonists to buy British goods, not French, Spanish, or Dutch products. In theory Americans would pay duties on imported goods to discourage this practice. The Navigation Act and Molasses Act were just two examples of royal attempts to restrict colonial trade.
Smuggling is the way the colonists ignored these restrictions.