system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] What would the imperium do with a "useless" medieval world?

Assuming the world has not a lot of resources, and uneducated medieval people that doesn't know a gun or a machine, what would they do with the world and the population after the world is brought to compliance?

Seems like a bad investment of time and people.
gpt
There's a line about how lasguns make no sound beyond the ionization of air.  Because of that, recruits from pre-gunpowder firearm societies have little trouble adapting to them as they're about as quiet as crossbows, bows, or thrown spears.  Same idea, same sound, slightly different execution - you're here, the enemy is there, you want them dead, do this - they're dead.

Those from firearm societies have problems adapting to them, as there's no loud explosion when they're fired.  They often have them modified in, as guns are supposed to be noisy.

Space Marines love recruiting from those backwards worlds as anyone able to survive to their teen years essentially already passed any physical tests needed to start the process to join their ranks.

They're also easy to bring in to compliance - an army of people waving regular swords around cannot stand against just a squad of Guardsmen.  Relative to high tech worlds, fewer resources in manpower are needed to bring the population in to compliance and, after a few lascannon demonstrations where a castle or two are vaporized or a particularly obstinate kingdom is flattened by orbital bombardment, the rest of the planet generally falls in line.

Even the worst performing world - crap land but not so crap that it can't feed the locals, no resources to speak of, harsh conditions but not so harsh that it's a deathworld, not even Space Marine chapters wanting to recruit from them - still provides people.

And the Imperial Guard needs people above all.  

You can train someone to fire a lasrifle in an afternoon.  The rest of the Guardsman life isn't too different than what they're already doing.