This 6' USB cable is used to charge and tether your Sprint, Verizon, Datajack, or Virgin Mobile MiFi. Not all USB cables are capable of both charging AND syncing data at the same time - this cable is!
Why do you need this cable? The Verizon MiFi comes with an 18" USB cable for tethering, but the Sprint version does not come with a USB cable at all. This cable has been tested with both versions and charges AND tethers the MiFi at the same time. The 6' length can also be beneficial as it allows you to position your device in a spot with better signal. Many users find that moving their MiFi away from their computer - by a window, for example - can help improve performance.
This cable can also be used to charge/sync other phones with Micro USB connections, such as the Blackberry Storm, LG Dare, LG Lotus, and many more.
A 3' cable is also available.
This is a charge AND sync cable. If you are looking for a CHARGE-ONLY cable (for example, if you want to be able to charge your MiFi via your computer without disabling the WiFi), you will want the retractable charge-only USB cable. Since the MiFi 4510L and 4082 cannot be tethered to a computer, users of those MiFis will also need the retractable charge-only USB cable.

The Sprint MiFi 2200 (sold separately) tethered with the 6' Micro USB cable