Orks allow
Gretchin to fight alongside them in
battle, or rather, in front of, to soak up
casualties and provide a
distraction. They can also run back to get more
ammo. Orks do, however, have other uses for the grots on the
battlefield. For example, the puny ones are great for
mine clearance. Just run the grots over the field to make the mines safe!
Gretchin are also used to man the ork artilery. While this may seem strange, remember that an ork loves nothing more than to get at the enemy up close and personal, so firing at the enemy at long range is very un-orkish.
Gretchin also often find themselves being "volunteered" for duty helping out ork mekaniks and Painboys (ork field surgeons), as either "Grot Orderlies" or "Grot Oilers"
Grots are also used by orks as a way to get better footing through difficult terrain (step on them).
As you've probably guessed, orks value gretchin very little, but "dat's OK 'cos dere's plenty left where 'dey came from."