Being a Grot is not a nice life. You are a scrawny, weak, little being among big, aggressive Orks who love to kill stuff and view anything smaller than themselves as inferiors to be yelled at and kicked, forced to do their drudge work and who's death is good for a quick laugh. Your life is one of cowering, hoping to avoid the wrath of your huge superiors, scraping up small scraps of food, doing what they say to hopefully avoid being kicked, eaten or shot; hiding and fighting with fellow grots over half-eaten squig legs. This often breeds spite and bitterness among many grots over the course of their often short lives.
But a precious few escape this fate. One day a Mekboy will scoop up some lucky grot, a painboy will hammer some wires into his brain and he will be sealed into a 3-meter-tall, armed and armored water-heater on legs called a Killa Kan. Suddenly he's bigger, harder, stronger and shootier than all those boyz who abused him, and it is payback time. After that, he gets to take out his latent frustrations on some spikey boyz, and everything's great for him. The only boyz he can't get after are those interred in Deff Dreds. Of course they're still every bit as cowardly as they were before, but that doesn't always matter when you think you're invincible.
Killa Kans are so favoured by Gretchin (I mean, who wouldn't love to control a hugeass war walker armed with all sorts of weaponry?) that they often hold lotteries whenever the resident Big Mek constructed a new one to find out who's going to get bolted in to pilot the thing. Only the actual wiring-in process can get most Gretchin to rethink their career decisions - as mentioned above, that's usually too late. Then again, once the process is actually finished, the Killa Kan is free to mutilate and kill his former tormentors, so that has to be worth something after all.
Don't ask how the grot feed themselves. Probably they dump some bits of those gits they carve up down the top hatch.