Gretchin occupy the very bottom of Ork society's brutal pecking order, smaller, weaker, and far less enthusiastic about dying than their towering Ork cousins. Where an Ork Boy charges into danger without a second thought, a Gretchin does everything in its power to avoid the same fate, and only the presence of an overseer wielding a wrangler's stick keeps most Gretchin mobs pointed in the right direction during a fight.
Despite their cowardice, Gretchin serve an essential function in Ork society as laborers, ammunition carriers, and expendable gun crews for weapons too large or unstable for a self-respecting Ork to want to operate personally. Their small size and nimble fingers make them surprisingly useful around delicate machinery, at least by Ork standards, and many a mekboy's contraption owes its continued operation to a terrified Gretchin frantically working the controls.
Strength in Numbers
What individual Gretchin lack in courage and combat skill, they compensate for through sheer disposability and numbers. Ork Warbosses view Gretchin losses with something close to indifference, treating them as a renewable resource to be spent absorbing enemy fire so that the real fighters can close the distance unharmed. It is a grim bargain the Gretchin never agreed to and cannot escape, bound as they are to serve greenskins who could crush them without a second thought.