The simple reason is that schedules change daily during the season. When you receive a new assignment or an assignment changes, you are supposed to receive an email from Arbiter. However, I have been told by many referees that it doesn’t always happen or that the email ended up in their spam folder. Therefore, the only way for you to be sure about your referee assignments is to log in to Arbiter and check the schedule.
The second big reason is that I need to know you have seen your schedule and that you will be at your assignment. Arbiter lets me know when you view your schedule. The status on your assignment changes from “Published” to “Accepted”. If I don’t see that your status has changed, I start the process to replace you with a different referee, if one is available.