I think it's just to give anything brewing time to actually show up (if it's going to) but in the end there really are no guarantees when you introduce any creature into a stable population. It's just not feasible to vet each and every breeder rat or your breeder/feeder efforts won't be cost effective. I don't always quaratine new rats, I do if I can though but every one of them gets a bath to get rid of that pet store stink. I think it also helps them lose their old colony smell so when they do go in with any of my rats, they are accepted better since they don't smell so different (no science behind this...just my own theory of how rat colonies work).