My understanding is that anytime you bring a bird in from anything other than a newly-hatched egg, there is the risk they may be carriers of one or more of diseases that harm chickens, that might possibly wipe out your flock. The new bird may look healthy and be asymptomatic, which is why you keep them away from your birds for 30 days. If they exhibit no symptoms after 30 days, it's considered safe to integrate them with your birds.
Quarantine is not perfect. If a carrier bird isn't stressed in the new environment, even a six week quarantine may not bring symptoms to the surface and you'll only know it was a carrier when you put it with the flock and the flock becomes ill. However, quarantining new birds is something I'd never go without, if I ever acquired started birds.