"Employing modern refining techniques, the Advanced Steel Mill produces Steel more quickly, and wastes less iron in the process. Replace two standard Steel Mills with one Advanced Mill, and save yourself some workers!"
Once you have an Advanced Steel Mill, keep it running as much as you can, because it is crucial to build yourself that tank battalion you always wanted. Not only that, but to keep those tanks running and in commission, you're going to need lots of steel.
The Advanced Steel Mill not only requires less time per job and produces more per hour, it does so more efficiently (i.e. greater output steel per input of iron and coal). Most players consider it well worth the nano cost.