The "point" is to setup the secondary character of the same class in the opposite way you setup the primary character of the same class.
IE, choose the alternate progression unlock item. That way you have access to all equipment, attatchments and equipment.
For example, have the stun mine on one Scout and the Claymore on the other. In addition, you might run out of attachment credits on the first character, but want to modify a weapon using different attachemtns or a weapon you didn't get the chance to modify.
As well as having fast access to the same class soldier that is setup completely differently. Rather than having to "waste time" modifying the character in the lobby or while the game is ongoing.
Aside from being able to start from scratch without having to respec your "first" guy.