We must realize that most actually town-concepts and public transport are build to seduce you to buy a car. (Mostly in western countries, where the car industry is one of the main successors.)
Oh the market is 40 min away and outside of the town and the bus only drives every two hours, well better buy a car or get lost.
Back to the 15-min Town, the concept is, there is everything you need for living (market, doctor, schools etc.) in an area you can reach in 15 min without a car.