For this question, let's just use your example of trader. Now our definition of trader would be the kind of trader that is also a merchant from the past era and not day trader as in a stock trader. Basically, a trader exchanges goods for money or any other form of monetary object. Now a trader can greatly contribute to the economic welfare of the city because he provides goods and services to the people of the city. He will receive money from the people in which he will have to pay taxes which can help contribute to the development of the city.