Not a stupid question at all. Sick or injured birds are at extremely high risk being killed by other flock members due to the fact it attracts predators. It is instinct, survival of the fittest. For this reason it is best to separate an injured bird until it is healed, it takes time to heal injuries. Birds that have respiratory diseases should be immediately separated and culled. The infected bird will spread whatever respiratory disease to other birds directly or indirectly.
Newly acquired birds should be kept separated a good distance away from an existing flock for 6 weeks. Newly acquired birds can introduce unseen bacterial, viral, fungal diseases, and parasites into an existing healthy flock if thrown together without a quarantine period. The further distance away from an existing flock, the better. Some diseases can be transmitted airborne such as Infectious Bronchitis. Diseases can be carried on your shoes, clothing, hands, vehicle tires etc...personal hygiene, changing clothes/shoes are paramount in preventing the spread of diseases. It's called 'biosecurity.'
Newly acquired birds that are quarantined gives most diseases time to show themselves from incubation time through course times. Then you can decide to treat or cull the birds. While new birds are quarantined; it gives you time to fully inspect them for injuries, external parasites, wormings, old wounds, and anything out of the ordinary that you would not want to introduce to your healthy birds.
As you can see, the definition of separation in the chicken world varies. It applies to the degree of separation that is required for the task at hand.