1. As a primary drive, enclosures allow computers to easily boot to XP, Vista, Windows 7, Linux and other OS's on the same computer.
2. Family members can boot have their own hard drive with their own OS and private data.
3. Corruption or failure of one hard drive would not affect the others.
4. As a secondary drive, enclosures can provide unlimited data storage. People who do lots of video and music editing and storage could benefit from a second removable hard drive. Second drive can be exchanged when full or for managing specific projects.
5. Physical Security: enclosed hard drives can be removed and stored away in safe place.
6. With multiple bays or where the unit is configured as secondary, removable enclosures can be used for high-speed data backups or restores for your primary drive.
7. Drive bay enclosures typically use a SATA or PATA interface, and are almost always faster than external Network, USB, or Firewire-enabled enclosures

A wide variety of internal and external racks and drive drawers are available to choose from. Some use carriers or caddies to protect the drive, while others just provide a simple way to install or replace a drive mounted in a drive bay. Pictures below are a representative example of many choices available at Micro Center.

 
 

CAUTION!

Remember to practice proper ESD prevention when handing drives with exposed circuit boards and connections. Keep loose drives in Anti-Static bags or blister enclosures to minimize risk. Use ESD ground straps to continuously discharge any static build-up prior to, or during handling. Avoid touching exposed circuitry or electrical contacts.

Remember, hard disk drives are sensitive to shocks and vibrations when spinning. Once the drive has stopped spinning (powered down), and the heads have had time to park, the risk is greatly reduced, but not eliminated.