When carbon and other materials are alloyed with iron it forms steel.
The amount of carbon is very small, it can be less than 1%.
Changing the amount of carbon, even by such small amounts, will dramatically change such properties as hardness, ductility and toughness.
Different properties mean different uses. Generally, low-carbon steels are used where toughness is needed.
Increasing the amount of carbon increases hardness, so high-carbon steels are used where hardness is needed.
It means that materials can be designed for special uses.