Via biggestharrypotterfans:

J.K. Rowling:

“The Sorting was some of the best fun I’ve had on this project. I was writing the potter books for sixteen years and during that time I had just had this real sense of where people belonged, in what house they belonged. It was something I was unconsciously doing a lot of the time when I met people.

“So, developing these vast pool of questions that are randomly selected for a user - so you wont get the same questions as your friend necessarily - I thought it was quite important that people didn’t get to second guess what meant Gryffindor, for example. But the exciting thing for me is that if you’re not sorted into Gryffindor, if you’re sorted into one of the other three houses you will effectively get an extra quarter chapter because you will go off to your on common room. If you are sorted into Gyrffindor you just follow Harry. But if you’re sorted into Hufflepuff, Ravenclaw or Slytherin, you go to your own common room, you meet your own prefect, who will tell you about famous people who were in your house and what the true nature of your house is.