Industrial Zone is a building zone which is needed by citizens to get their job/career, like commercial zones. They also produce the goods your city needs, which is why buildings in the industrial zone are factories and cause pollution. Edit
Industrial Zone is needed for developing cities/towns as it is required by citizens to get their job/career.
Without an Industrial Zone towns/cities never grow well, resulting in town/city bankruptcy. Industrial Zone is also very helpful for production/distribution activities in the town/city.
Once the City/Town you build has reached 5,000 inhabitants, it's OK to reduce Industrial Zone, this zone can be replaced with non-polluting Commercial zone.
Numbers of workers in Industrial Zone is different depending on it's density, dense industrial can open up to 500 Employment,
The Maximum Capacity of an Industry is 729 Employment(According in the game file)