With an account you can:
easily check the items you booked
download your ticket
download your invoice again any time
download your certificate of attendance after the event
Later on, you will also be able to manage your preferences regarding participating in the various social events, etc.
If you come back to Agile Testing Days USA another time,
you automatically get recognized as Alumnus/a, plus you save time during registration and only need to check if anything needs updating.