The whole idea of mobile racks is simple. It's removable storage.
Instead of mounting an internal drive which is permanent, you could use mobile racks which allows you to make swap outs without fuss.
The applications are varied. If you look at my signature you will see that I run several Operating Systems. I do not like to dual boot. I prefer the clean isolation so I use a different hard drive mounted in a mobile rack. This allows me to swap out the racks depending on which Operating System I choose to run.
The rack comes in two pieces. A frame which is permanently mounted in the PC and a tray (contains the hard drive) which is inserted and removed from the frame. The trays can be purchased seperately, thus the $10 price tags you may encounter.
Variations in price reflect varied features such as size, cooling, construction, speed (ATA100/133 5400/7200RPM), interface (PATA/SATA, SCSI), hot swappable (OS supported), etc,.