How was the British policy of salutary neglect good for both the British King and the American colonies?

Britain's unwritten and unofficial policy of relaxing trade regulations on its colonies, which became known as salutary neglect, did indeed have a positive effect on both sides of the Atlantic. In the 1720s, when Parliament decided to relax enforcement of the Navigation Acts, the colonists began to trade more in non-British goods and with non-British entities. This allowed a flowering in a trade that was technically illegal, but mostly unenforced.
Without the restrictions of the Navigation Acts, the American colonies prospered as more money from commerce flowed in. Merchants from the colonies would trade directly with the French, Dutch, and sometimes the Spanish, bypassing English middlemen. It opened up the possibility to trade in a wider array of raw materials and goods with a more diverse range of parties. This was a boon for the economy of the colonies, particularly in the trading hubs of New England.
This relaxation of trade restrictions ended up benefiting England as well. With more wealth in the colonies, the colonists were able to spend their money on more British goods and products. Ironically, allowing the colonists to trade with non-British entities resulted in a more profitable trade with Britain.
A further benefit of salutary neglect for Britain was that they stopped spending the resources on enforcing the Navigation Acts. Smuggling became profitable almost immediately after the Navigation Acts were put in place. Effectively combating this took up a considerable amount of resources. By easing enforcement, the British were able to focus their attention on important matters closer to home.
https://historyofmassachusetts.org/what-was-the-british-policy-of-salutary-neglect/