From Destinypedia, the Destiny wiki
Initially, a Factory Walker is deployed to battle the Guardian, but is crushed by the Factory Brig to initiate the final boss encounter. The Brig, however, would be destroyed and the sabotage of the factory be completed.
Apart from design and appearance, they have the same gameplay as regular Brigs apart from some of their attacks being intensified such as their Ground stomp/slam attack.
List of appearances