Cats are meant to be social creatures, but we only have so much time in the day to spend socializing with them. A second kitten is a 24/7 playmate and cuddle partner. Adopting in pairs ensures they always have someone (besides you) to pounce on at 4AM, and to socialize with while the humans are working.
Another kitten teaches how to play nicely without scratching or biting. A cat with a friend will be less needy and lonely, but will still love you just as much. Just like with people, it is healthier to have a social circle beyond one person. It can be hard to add a second animal later, which is why we recommend adopting as a pair to start with.