What did the colonies have to gain by declaring independence?

By declaring independence from Britain, the colonists gained freedoms that they did not have while under British control. By writing the Declaration of Independence, the colonists made the argument that each person possess the unalienable rights of "life, liberty, and the pursuit of happiness". This patriotic way of thinking helped colonists win the war against Britain once the revolution began. The colonists also received economic freedom by declaring their independence. Colonists were free from taxation that they believed the British unfairly placed on them. They also were free to trade with other countries besides Britain. This allowed for more independence to trade with who they wanted, and allowed them better prices. The declaring of independence also lead to a new philosophy on government rule which influenced the Constitution and many of the decisions the founding fathers made. They viewed that the people must have a say in government, and the government must protect the rights of the citizens. They viewed the King George III as a tyrant with too much power, and made sure that when setting up their new government, to have a system of checks and balances to not put too much power in one persons hands.

The colonies were gaining more self-governance for themselves by declaring independence from Britain. The colonists in favor of separation from the mother country wanted to be represented by their own leaders. They wanted to ship American goods using American vessels to anyone they wished. They also wanted American criminals to be tried by juries of their peers rather than British admiralty courts as was the situation with smugglers under the British system of government. The colonists also wanted to remove the presence of British soldiers who could inspect property any time they suspected smuggling. Many colonists also did not want to support the Church of England through taxation.
The colonists actually had more to lose by declaring independence. Britain was the largest buyer of American products. The British navy and army provided protection against foreign powers such as the Spanish and the French. This was a concern had by many on the eve of the Revolution—there was quite a bit of uncertainty tied to attaining nationhood for the American colonies.