Mines take more work to produce iron than collecting it from the landscape. However it is the only way to produce iron - other than through trading - after the loose ore on the ground has been collected.
Mines have a limited amount of material that can be removed from them. Once material is gone, the mine will no longer be useable. Mines cannot be fully removed. The building can be taken down, but the area that the mine takes up won't ever be available for use for other purposes.
Mines are dangerous - the chance of workers dying in a mining accident is higher than any other profession. Educated citizens have a reduced chance of dying in a mine.