The reason why you should quarantine a new Koi is that it could possibly have a parasitic or bacterial infection. Your new Koi could even have a virus. If you new Koi has one of those ailments, it will be much easier to treat it in a small hospital tank than in a pond. Quarantining new Koi is cheap insurance for keeping your other Koi healthy and alive. If something does afflict your new Koi, there is a very little chance for it to spread to your Koi that are already in your collection.
New Koi should be quarantined for 3-4 weeks. Usually, anything that's going to afflict them will appear in that amount of time.
A good quarantine tank will be large enough for your new Koi to swim in and will have a good filter system.