Buying a baby bunny is something that most 4H-ers and people starting out love to do. This can be an amazing experience because you get to raise your rabbit from a young age and watch it grow. You are able to handle your baby in the same way it will be as an adult which allows for it to get used to the life it will have at a more impressionable age.
People feel very strongly that bonding with a younger rabbit is an easier and faster process. While I definitely agree that bonding with a baby is easier than bonding with a rabbit during puberty, I think bonding with an adult depends entirely on the personality of the rabbit, but it is still a consideration for many people considering buying a baby.
Litter box training may be easier. I know a lot of people, especially starting out really love to have litter trained rabbits and it is reportedly better to start litter training your rabbit before the age of 4 months old for the best long-term success.
Babies are more active. I could spend hours just watching my babies play. They are curious about everything and the whole world is new for them. Babies are great entertainment.
You also get to see firsthand how your breed develops. Purchasing a baby will ALWAYS be a learning experience.