Usually it asks you where to mount root from because you have changed the system configuration somehow, like removing the cd-rom drive, and the fstab is now pointing at the wrong drive.
For example you have the CD-ROM and HD on the same IDE channel using cable select. In this config the HD is the primary slave, ad1, so that's what gets set in the fstab. When you remove the cd drive the HD now becomes the primary master, ad0, so the fstab must be changed.
Steve