Why is iron often used to alloy steel?

Iron is the basis of steel - from 95 to 99% of it. It is what is added to the iron to make various types of steel. In virtually all steels, the first thing added is carbon, anywhere from a speck to about 2.5%. Other common additions to the iron to make other kinds of steel are manganese, nickel, chromium, vanadium - alone or in various combinations, depending on the alloy desired.
There are many, many different alloys of steel.