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. That's when the kitty will learn every nook and cranny, test the opportunities to escape and begin to associate the arrival of humans with food and low-key pleasurable attention. Each successful brief human visits builds confidence. The kitty may need to hide at first, watching as food is delivered and the litter box is cleaned, but as that 48 hours ends, there should be some brief and tentative interaction.