1. You bought the bike because it was the most most comfortable ride you could find while still having that fun race bike feeling (I did this).
2. You want to use your bike for multiple purposes - you take the rack off for racing, but also want to be able to tour on it or commute to work on it (like where I am we have security watching the parking lot and bike racks), or you ride the bike across town and want to be able to carry shoes for when you get there.
3. You want to be able to carry basic necessities while training for racing. Rain gear is very uncomfortable to wear while riding normally, but you don't want to be stuck 50 miles from home when it starts raining and it's 35 degrees out either because, you know, you don't want to get hypothermia.

Most people cannot afford to hire a team to follow them around in a car with the necessities every time they go out on a ride on their bike. ;-)