The cables from the Connector Board to the Motherboard are all designed to be unplugged from the Connector Board. This means that you can connect them to the Motherboard while it is out of the case, when it is much easier to get to them; and then leave them permanently in place.

The Connector board is not only more accessible than headers on the motherboard, but also uses locking Connectors to prevent any of the cables from being inserted incorrectly.