The reasons for this are manifold but there is an ongoing effort to tie in a user’s Apple ID with all of their actions in Apple’s ecosystem. This new procedure is in place for Genius Bar and Workshop appointments and there is now a profile page that allows you to see all of your appointments past and present.
The site says the change is also to "give the control of a Genius Bar appointment to a user directly, who must know their credentials and enter them before being able to confirm a payment."
Another factor may be the time savings. Instead of having to enter a user's information when they arrive for the appointment it will already be populated.