Developers normally do not concern themselves with the local economy. They build industrial and commercial space when the economy is growing and business is buying. They build housing when the economy is attracting new buyers - either because businesses are hiring, interest rates drop or people are making more money so they can buy new homes. That system is based on transport, and as growth occurs, the roads become congested and a whole host of new problems crop up.
In contrast, a Market Town focuses on creating a new local economy that is not dependent on regional growth. In fact, a Market Town can become an economic engine for a weak regional economy so long as the growth factors for the town are present. These include access to ultra-fast broadband (fibre optic), reasonable access to a major airport when long-distance travel is needed, and the basic requirements of water, a pleasant climate, topography, law and peace.
The Market Town is a mixed use settlement. This means that small enterprises are run in the villages. Every home is permitted to have a home office. On primary streets, the businesses can have significant foot traffic and deliveries. The busiest businesses will be on the village plazas. Medium-sized, white collar businesses operate in the 3 and 4 story office buildings in the cosmopolitan town centre. Blue collar businesses, that are inappropriate near where people live, are run in the walk-to industrial park. Most of these will be 21st century, clean-room industries that use high-tech manufacturing.
Instead of focusing on selling homes, the Market Town focuses its efforts on attracting businesses and jobs. At least 20% need to sell local-to-global (L2G) meaning that they are fibre-based businesses that could locate anywhere there is ultra-fast broadband. They will move to the Market Town because of the other attractions. These are the money importing businesses. When that money becomes income for the people who own or work in those businesses, it then is spent locally. This is called the multiplier effect, or money-turn.
This is where critical mass comes in. This is why the Market Town needs 5,000 to 10,000 people. Businesses that sell Local to Local (L2L) need customers. Local incomes include service workers, teachers, professions, shops, recreation and so on. Some are essential, others discretionary. Some are hybrids, such as those serving the visitor industry. The income comes from outside, but the goods and services are sold locally. An attractive Market Town can be expected to have a major visitor industry that operates year round.