The benefit to choosing a hard drive case, then putting your own hard drive into it is simple. When you pick an external hard drive, you get whatever drive the manufacturer decides to put in the enclosure. When you buy your own, you can pick the best hard drive you can afford to go with your system.
TIP! The most common hard drive case is USB, if you can go with eSATA or Firewire if your computer has a port that lets you use one for added speed when moving files.
When you pick out a hard drive case, try and get something that is versatile. The one pictured above has eSATA as well as USB. This way if your computer has eSATA you can use that interface because of the speed gains, if not, use the USB which is the more common interface that just about any computer will have. If you do a little research, you can probably find one that has the 2 most common ones that you will use.