From Lexicanum

Brute Squads are comprised of the bigger and nastier Orks in a Feral Ork tribe. They spend their time maintaining order within the hierarchy of the tribe, usually by means of their sheer size and bullying tendencies. They allow the other smaller Orks the privilege of hunting for them, taking the best of the food that the Huntas bring back.
Whilst the ability to shoot prey is essential to a Feral Ork tribe's survival, they still relish closed quarters combat.

Brutes are also used as Minderz by the Warboss. They have special orders to keep an eye on a Wyrdboy to make sure he does his job in battle. In such circumstances a Brutes bullying comes in to its own.

Sources

• ¹: Chapter Approved 2003: Feral Orks, pp.4-10