It's dirt cheap and incredible versatile. It has many different crystal structures that can give you an incredibly wide range of properties. General purpose mil

It's dirt cheap and incredible versatile. It has many different crystal structures that can give you an incredibly wide range of properties. General purpose mild steel used in buildings. Stainless steel that resists corrosion. Maraging steels used in aerospace. Also, the high modulus is interesting. Some components are stiffness limited such that you couldn't use aluminum or titanium even if you wanted.