Notebook drives are relatively expensive for their storage size, so being able to reuse a PowerBook drive is a big plus in my opinion. By building from a kit you'll also know how to replace the drive if you want to upgrade it in the future. Satisfaction of doing it yourself is another reward. For instance if your Powerbook needs an internal hard drive upgrade, you can reuse the existing drive in the kit's case. Lower cost than some pre-assembled drives if you build it yourself.
Savings varies by drive size.
Easy to upgrade in the future to larger drives. Excellent way to backup or offload the internal drive for infrequently used files. Hot plug capability. Sensitive or private files can be locked away and not stored on the internal drive. Portable drives cost more per GB than full size external firewire versions.