You might have some users that currently do not have emaill addresses within your company but the MX user setup screen is requiring that this be populated.  

It is stressed that you create email addresses for all of the users so that you can get the full use out of the systems Triggers, notifications, etc. . . .

Without this in place then you would have to resort to manually telling them this information rather than streamlining the process a bit by utilizing our Triggers and notifications.