In order to have a flame, you need fuel oxygen and a source of ignition. The air we breathe contains about 20% oxygen. Quite bluntly, the pressurised oxygen in a typical torch setup (except for cutting) allows you to burn more fuel with a proper ox./fuel ratio than you could by simply feeding air to your flame. So basically, you could use regular air but, you would need a different torch and different pressures, wich would give you a far inferior Heat output. Now for cutting, even if you reached the kindling temperature, you would not be able to oxydise and cut the iron.