During the Industrial Revolution, the center of the economy shifted away from agriculture and towards manufacturing. Manufacturing, as it became increasingly large and important, located itself in large factories, employing hundreds of employees in a single location.
Factories, though not wonderful places of employment, often offered better wages than agricultural work, so laborers flocked to them. As workers gathered in towns and cities to find factory work, these areas grew to service worker needs for housing near the factories, for food they could no longer grow themselves, and for other necessities. Early factories, such as textile factories, often clustered in a single location near raw materials, such as wool, or the water the needed to help power their industries. These factory clusters, such as in the north of England (sheep country) in places like Manchester, led to rapid urban growth.
England was the first country to industrialize in a large-scale way, primarily around the textile industry. As the first to experience a massive shift from an agrarian lifestyle centered around small villages to populations aggregating in cities, England simply wasn't prepared for the change. Conditions were often appalling around the factories, but over time, despite the sometimes terrible conditions, industrialism improved the standard of living. Cheaper cloth meant people could buy far more clothing, helping to prevent the spread of disease and giving people more protection against the cold. Even the poor often ate better than they had on the farms, leading to increases in family size. At the higher echelons, people gained great wealth or prospered as a rising middle class. This also expanded cities and towns as increasing wealth led people to build bigger houses, buy more goods, and want access to more leisure activities. In a post-industrial world, people continued to like and desire the amenities and conveniences associated with urban life.