Basically to drive down the cost of payload mass as much as possible. First you have to build a lot of rocket to get anything up into space. If it only launches

Basically to drive down the cost of payload mass as much as possible. First you have to build a lot of rocket to get anything up into space. If it only launches 1kg of payload, that's going to be a very expensive kilogram. As you continue to scale the rocket up, assuming you can design for a decent payload ratio, the amount of cargo capacity increases and the cost per kilogram drops.