Nobz are what happens when an ordinary Ork Boy simply keeps winning fights until nobody in the mob is left willing to challenge him. Larger, tougher, and considerably better armored than the Boyz around them, Nobz occupy the natural leadership tier of Ork society, not through cunning or charisma but through an unbroken record of physically dominating anyone who questioned their authority.
A mob without a Nob to keep it organized tends to devolve quickly into a directionless brawl, so most Warbosses make sure every unit under their command has at least one Nob riding herd on the smaller Boyz. Nobz relish the role, since it grants them first pick of looted weapons and armor, and most decorate themselves with a growing collection of trophies, scars, and cobbled-together plate that makes each one distinct from the mob he leads.
Ambition Wrapped in Muscle
Every Nob secretly measures himself against the Warboss he serves, and more than one Ork chieftain has met his end at the hands of a Nob who decided he was finally strong enough to take over. This ever-present threat of internal challenge keeps Ork hierarchies brutally honest: a Warboss who slows down or shows weakness rarely stays in charge for long, and his Nobz are usually the first to notice and the first to act on it.