As a conference guest, specifying your timezone will let you see when forms are due for the timezone you are in. You may be in a different timezone than the conference admin. When they set a due date for the forms, it will be in the timezone they're in. This helps keep everyone on the same page.
Similarly for an admin, the timezone determines when the forms are due. If your guests are in a different timezone, they'll see the forms due at the appropriate time for them that matches with your own timezone.