You do not HAVE to use a B2D folder, however if the Backup with GRT enabled is on a tape drive, you will need a 'staging' area that contains enough space for the backup set + the restore. See notes below from the Backup Exec 12.5 Help section.
Tape:
Backup Exec copies the entire backup set or sets to the staging area. The staging area must have enough disk space for the entire backup set or sets from which you want to restore an individual item.
Before you use a tape device for a GRT-enabled backup, ensure that sufficient disk space is available to perform a restore.
Backup Exec deletes the data from the staging area when the restore job is complete.
