How to calculate the weight of steel flat bars and Circular rods?

Why we have to calculate the weight of steel and why we have to derive the weight of steel bars formula? For this you need to understand how estimation works, for every project the steel is represented in length as meter, foot, etc,… and diameter of the rod in mm, for example, we estimate steel as 10 meters of 8mm diameter bar or 10 foot of 8mm bar, etc…