Adding transient suppressors to USB devices [PDF!] can help prevent damage from static discharge. This is important because USB devices are handled without protection, carried in pockets, tossed in bags, etc.
Because frequent human interaction with the USB system occurs as a result of its attractive hot-plugging ability, there is the possibility for large ESD strikes and damage to crucial system elements. The ESD protection included on the existing hardware is typically in the 2-kV to 4-kV range for the human body model (HBD) and 200-V to 300-V for the machine model (MM). The ESD voltage levels found in a normal USB operating environment can exceed these levels.