Physical punishment simply won't work with cats. Unlike dogs, they cannot be persuaded that a human is the leader of the pack. In fact, in nature, they do not relate as a pack. Colony life is based on respect for one another's skills and times of privacy as well as socializing. The greatest compliment a cat can give to a human is recognition as a colleague.
Infectious diseases can spread like wildfire.
In a multi-cat household there will be curiosity, but the quarantine reduces stress for the new cat and reduces the level of confrontation when there is an introduction. The new cat will, over time, regard the quarantine area as a place of safety if other cats are overwhelming. Quarantine is particularly helpful in socializing a feral kitten or cat. It allows the kitty to master the territory, learn to accept human visits that respect kitty territory and limit the number of stimulation's the kitty has to deal with in adjusting to a new environment. The first 48 hours tend to be critical.