if you have to ask "why?", then it's not for you
Having 100 cables lengths means professional multi-room installations are no longer hampered by length-restricted consumer cables like HDMI, USB, Firewire, etc. For example, in a recording studio, the computers and terabytes of storage are typically in a separate sound proof machine room, then there are monitors and keyboards in the control room where the engineer sits, and more monitors and more USB inputs in the tracking room where the artist (e.g. in film the artists need to see whats on the screen). The distances between the rooms are typically more than USB and HDMI will go, so in the past you end up with repeaters, or converting to ethernet and then back to USB/HDMI, or whatever. Now you can just put a thunderbolt USB/HDMI doc in each room, connected with a 100' thunderbolt cable - easy, and reliable, and compared to the alternatives, cheap.
If you just have a hard drive sitting next to your iMac, then no, this isn't for you.