We do it because of two purposes.
The first is fairly mundane. It is because we need to tie the payment to the athlete registration. This ensures if there are any issues we have proper contact information to resolve them.
The second is just as important though. We do it so that the ongoing user experience is as easy and rich as possible.
Once you create a user account, you can add 1 or more athletes under that user. We want to support parents of one all the way up to teams of 25 or more under a single parent or coach user. After adding all the athletes, registering for tournaments is a simple as selecting the events you want for each athlete.
You only create an athlete once. Afterward, you can register that athlete for any tournament run on TourneyGo. You only have to update weight and rank when needed. Plus the system provides each athlete with a complete history for all tournaments in which they participated.