From Command and Conquer Wiki

The Mobile War Factory was introduced after the Second Tiberium War to allow GDI commanders to produce units right where they needed them, close the the battle. The unit was about as heavy as a Mobile Construction Vehicle so it could only move very slow around the battlefield. Also it needed to be escorted as it was not armed. However, it did have heavy armor which could withstand a smaller attack. GDI could use Orca Carryalls, to move these about the battlefield, ensuring a steady supply of replacement units whenever they went into battle, which made GDI attacks all that much more forceful.

When deployed, it was a fully functional war factory capable of producing all units in the GDI arsenal, except those that required additional support structures, such as a Tech center.

Mobile War Factories were defenseless, but the key was always getting to them quickly. The longer they were allowed to operate, the larger the probability that they would produce more units. If they produced more units, these would be units that would have to be dealt with to get to the Mobile War Factory. Furthermore, Mobile War Factories deployed as support to attacks on Nod bases could be extremely difficult to get to, as the attack force would have to go through the enemy strike. After the Firestorm Crisis, the Mobile War Factory was considered cost-ineffective and latter phased out.