The business of Rome is business. Without farms, workshops, markets and related buildings, your city will never grow much past hovels, and no one will want to move in.
Commerce is a prime source of both jobs and, through trade, money.
Market - Granaries bursting with food, and warehouses overflowing with goods, are useless without markets to distribute these goods to homes. Teenaged Romans like to hang out at the market. Clay Pit - Here you can find glamorous jobs digging clay to obtain raw material for pottery. Pottery Workshop - Modern living in highrise insulae would be impossible without pottery vessels to move and store life's essentials. Farm - Food is a top priority in any city, and it's better to grow it than to import it.