It allows your bacterial levels in your tank to acclimate to the new amounts of waste that is going into the system. Your tank will support a certain level of bacteria with just one fish to reach equilibrium. Add another fish and the bacteria has to adjust again, and so on. Add several fish simultaneously and it's a huge adjustment. Should take this opportunity to set up a QT. The QT allows you to not only watch your new fish for diseases, but also gives you and the fish time to get used to one another, feeding times and type of food, ability to eat without a bunch of competition from other fish, etc. I agree with Lisa.